feat(mobile): Nostr identity — Amber NIP-55 deep link + nsec fallback

Implements mobile Nostr identity management per issue #29.

Android — NIP-55 Amber integration:
- Opens com.greenart7c3.nostrsigner via `nostrsigner:` URI scheme to
  retrieve the user's public key without exposing it to the app.
- Listens for the `mobile://nostr-callback` deep link response and stores
  the resulting npub in Expo SecureStore.
- Falls back to Play Store install prompt when Amber is not installed.

iOS / manual fallback:
- NostrConnectModal accepts an nsec1 paste-in, validates bech32, derives
  the pubkey via nostr-tools getPublicKey, and stores the key only in
  Expo SecureStore — never in AsyncStorage, Redux, or logs.

Both platforms:
- Truncated npub and signer type (Amber / nsec) shown in Settings.
- "Disconnect Nostr" wipes all keys from SecureStore and resets state.
- Identity persists across restarts via SecureStore.

Supporting changes:
- NostrContext: new React context for identity lifecycle.
- NostrConnectModal: platform-aware bottom-sheet modal for connect flow.
- TimmyContext: added apiBaseUrl/setApiBaseUrl/isConnected; URL persisted
  in AsyncStorage and restored on mount; circular dep broken via refs.
- constants/colors: added field, textInverted, destructive, link colours.
- constants/storage-keys: added SERVER_URL_KEY.
- app.json: added Android intent filter for mobile://nostr-callback.
- package.json: added nostr-tools and expo-secure-store dependencies.

Fixes #29

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/mobile/context/TimmyContext.tsx

@@ -8,6 +8,9 @@ import React, {
   useState,
 } from "react";
 import { AppState, Platform } from "react-native";
+import AsyncStorage from "@react-native-async-storage/async-storage";
+
+import { SERVER_URL_KEY } from "@/constants/storage-keys";
 
 export type TimmyMood = "idle" | "thinking" | "working" | "speaking";
 
@@ -22,33 +25,42 @@ export type WsEvent = {
   count?: number;
 };
 
-export type ConnectionStatus = "connecting" | "connected" | "disconnected" | "reconnecting" | "error";
+export type ConnectionStatus =
+  | "connecting"
+  | "connected"
+  | "disconnected"
+  | "reconnecting"
+  | "error";
 
 type TimmyContextValue = {
   timmyMood: TimmyMood;
   connectionStatus: ConnectionStatus;
+  /** True when the WebSocket is fully open */
+  isConnected: boolean;
   recentEvents: WsEvent[];
   send: (msg: object) => void;
   sendVisitorMessage: (text: string) => void;
   visitorId: string;
+  /** Current API / WebSocket base domain */
+  apiBaseUrl: string;
+  /** Persist a new base URL and reconnect the WebSocket */
+  setApiBaseUrl: (url: string) => void;
 };
 
 const TimmyContext = createContext<TimmyContextValue | null>(null);
 
 const MAX_EVENTS = 100;
-const BASE_URL = process.env.EXPO_PUBLIC_DOMAIN ?? "";
+const ENV_DOMAIN = process.env["EXPO_PUBLIC_DOMAIN"] ?? "";
 const VISITOR_ID =
   Date.now().toString() + Math.random().toString(36).substr(2, 9);
 
-function getWsUrl(): string {
-  let domain = BASE_URL;
-  if (!domain) {
-    domain = "localhost:8080";
-  }
-  domain = domain.replace(/^https?:\/\//, "");
-  domain = domain.replace(/\/$/, "");
-  const proto = domain.startsWith("localhost") ? "ws" : "wss";
-  return `${proto}://${domain}/api/ws`;
+function buildWsUrl(domain: string): string {
+  let d = domain.trim();
+  if (!d) d = "localhost:8080";
+  d = d.replace(/^https?:\/\//, "");
+  d = d.replace(/\/$/, "");
+  const proto = d.startsWith("localhost") ? "ws" : "wss";
+  return `${proto}://${d}/api/ws`;
 }
 
 function deriveMood(agentStates: Record<string, string>): TimmyMood {
@@ -63,10 +75,12 @@ function deriveMood(agentStates: Record<string, string>): TimmyMood {
 }
 
 export function TimmyProvider({ children }: { children: React.ReactNode }) {
+  const [apiBaseUrl, setApiBaseUrlState] = useState(ENV_DOMAIN);
   const [timmyMood, setTimmyMood] = useState<TimmyMood>("idle");
   const [connectionStatus, setConnectionStatus] =
     useState<ConnectionStatus>("connecting");
   const [recentEvents, setRecentEvents] = useState<WsEvent[]>([]);
+
   const wsRef = useRef<WebSocket | null>(null);
   const retryTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
   const retryCountRef = useRef(0);
@@ -77,6 +91,32 @@ export function TimmyProvider({ children }: { children: React.ReactNode }) {
     delta: "idle",
   });
   const speakingTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+  // Stable ref so WebSocket callbacks always read the current URL
+  const apiBaseUrlRef = useRef(apiBaseUrl);
+  // Stable refs to break the connectWs ↔ scheduleRetry circular dependency
+  const connectWsRef = useRef<() => void>(() => {});
+  const scheduleRetryRef = useRef<() => void>(() => {});
+
+  // ── Load persisted URL on mount ────────────────────────────────────────
+
+  useEffect(() => {
+    AsyncStorage.getItem(SERVER_URL_KEY)
+      .then((stored) => {
+        if (stored) {
+          setApiBaseUrlState(stored);
+          apiBaseUrlRef.current = stored;
+        }
+      })
+      .catch(() => {});
+  }, []);
+
+  const setApiBaseUrl = useCallback((url: string) => {
+    setApiBaseUrlState(url);
+    apiBaseUrlRef.current = url;
+    AsyncStorage.setItem(SERVER_URL_KEY, url).catch(() => {});
+  }, []);
+
+  // ── WebSocket helpers ──────────────────────────────────────────────────
 
   const addEvent = useCallback((evt: Omit<WsEvent, "id" | "timestamp">) => {
     const entry: WsEvent = {
@@ -94,14 +134,14 @@ export function TimmyProvider({ children }: { children: React.ReactNode }) {
       wsRef.current.close();
       wsRef.current = null;
     }
-    const url = getWsUrl();
+    const url = buildWsUrl(apiBaseUrlRef.current);
     setConnectionStatus("connecting");
     let ws: WebSocket;
     try {
       ws = new WebSocket(url);
     } catch {
       setConnectionStatus("error");
-      scheduleRetry();
+      scheduleRetryRef.current();
       return;
     }
     wsRef.current = ws;
@@ -134,10 +174,7 @@ export function TimmyProvider({ children }: { children: React.ReactNode }) {
 
       if (type === "world_state") {
         const states = (msg.agentStates as Record<string, string>) ?? {};
-        agentStatesRef.current = {
-          ...agentStatesRef.current,
-          ...states,
-        };
+        agentStatesRef.current = { ...agentStatesRef.current, ...states };
         setTimmyMood(deriveMood(agentStatesRef.current));
         return;
       }
@@ -187,7 +224,7 @@ export function TimmyProvider({ children }: { children: React.ReactNode }) {
 
     ws.onclose = () => {
       setConnectionStatus("disconnected");
-      scheduleRetry();
+      scheduleRetryRef.current();
     };
 
     ws.onerror = () => {
@@ -200,9 +237,15 @@ export function TimmyProvider({ children }: { children: React.ReactNode }) {
     const delay = Math.min(1000 * Math.pow(2, retryCountRef.current), 30000);
     retryCountRef.current += 1;
     retryTimerRef.current = setTimeout(() => {
-      connectWs();
+      connectWsRef.current();
     }, delay);
-  }, [connectWs]);
+  }, []);
+
+  // Keep the stable refs current after every render
+  connectWsRef.current = connectWs;
+  scheduleRetryRef.current = scheduleRetry;
+
+  // ── Initial connect ────────────────────────────────────────────────────
 
   useEffect(() => {
     connectWs();
@@ -216,7 +259,19 @@ export function TimmyProvider({ children }: { children: React.ReactNode }) {
     };
   }, [connectWs]);
 
-  // AppState-aware WebSocket reconnect on foreground
+  // Reconnect when apiBaseUrl changes (skip the very first render)
+  const isFirstRenderRef = useRef(true);
+  useEffect(() => {
+    if (isFirstRenderRef.current) {
+      isFirstRenderRef.current = false;
+      return;
+    }
+    retryCountRef.current = 0;
+    connectWs();
+  }, [apiBaseUrl, connectWs]);
+
+  // ── AppState-aware reconnect on foreground ─────────────────────────────
+
   useEffect(() => {
     if (Platform.OS === "web") return;
 
@@ -229,20 +284,17 @@ export function TimmyProvider({ children }: { children: React.ReactNode }) {
       const isNowActive = nextAppState === "active";
 
       if (wasBackground && isNowActive) {
-        // App returned to foreground — check if WS is still alive
         const ws = wsRef.current;
         if (!ws || ws.readyState !== WebSocket.OPEN) {
-          // Cancel any pending retry so we don't create duplicates
           if (retryTimerRef.current) {
             clearTimeout(retryTimerRef.current);
             retryTimerRef.current = null;
           }
           retryCountRef.current = 0;
           setConnectionStatus("reconnecting");
-          connectWs();
+          connectWsRef.current();
         }
       } else if (nextAppState === "background") {
-        // Proactively close the WS to avoid OS killing it mid-frame
         if (retryTimerRef.current) {
           clearTimeout(retryTimerRef.current);
           retryTimerRef.current = null;
@@ -262,7 +314,9 @@ export function TimmyProvider({ children }: { children: React.ReactNode }) {
     return () => {
       subscription.remove();
     };
-  }, [connectWs]);
+  }, []);
+
+  // ── Outbound messages ──────────────────────────────────────────────────
 
   const send = useCallback((msg: object) => {
     if (wsRef.current && wsRef.current.readyState === WebSocket.OPEN) {
@@ -270,28 +324,42 @@ export function TimmyProvider({ children }: { children: React.ReactNode }) {
     }
   }, []);
 
-  const sendVisitorMessage = useCallback(
-    (text: string) => {
-      if (wsRef.current && wsRef.current.readyState === WebSocket.OPEN) {
-        wsRef.current.send(
-          JSON.stringify({ type: "visitor_message", visitorId: VISITOR_ID, text })
-        );
-        setTimmyMood("thinking");
-      }
-    },
-    []
-  );
+  const sendVisitorMessage = useCallback((text: string) => {
+    if (wsRef.current && wsRef.current.readyState === WebSocket.OPEN) {
+      wsRef.current.send(
+        JSON.stringify({
+          type: "visitor_message",
+          visitorId: VISITOR_ID,
+          text,
+        })
+      );
+      setTimmyMood("thinking");
+    }
+  }, []);
 
-  const value = useMemo(
+  // ── Context value ──────────────────────────────────────────────────────
+
+  const value = useMemo<TimmyContextValue>(
     () => ({
+      timmyMood,
+      connectionStatus,
+      isConnected: connectionStatus === "connected",
+      recentEvents,
+      send,
+      sendVisitorMessage,
+      visitorId: VISITOR_ID,
+      apiBaseUrl,
+      setApiBaseUrl,
+    }),
+    [
       timmyMood,
       connectionStatus,
       recentEvents,
       send,
       sendVisitorMessage,
-      visitorId: VISITOR_ID,
-    }),
-    [timmyMood, connectionStatus, recentEvents, send, sendVisitorMessage]
+      apiBaseUrl,
+      setApiBaseUrl,
+    ]
   );
 
   return (