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/app/_layout.tsx

@@ -16,6 +16,7 @@ import { SafeAreaProvider } from "react-native-safe-area-context";
 
 import { ErrorBoundary } from "@/components/ErrorBoundary";
 import { TimmyProvider } from "@/context/TimmyContext";
+import { NostrProvider } from "@/context/NostrContext";
 import { ONBOARDING_COMPLETED_KEY } from "@/constants/storage-keys";
 
 SplashScreen.preventAutoHideAsync();
@@ -77,9 +78,11 @@ export default function RootLayout() {
         <QueryClientProvider client={queryClient}>
           <GestureHandlerRootView style={{ flex: 1 }}>
             <KeyboardProvider>
-              <TimmyProvider>
-                <RootLayoutNav />
-              </TimmyProvider>
+              <NostrProvider>
+                <TimmyProvider>
+                  <RootLayoutNav />
+                </TimmyProvider>
+              </NostrProvider>
             </KeyboardProvider>
           </GestureHandlerRootView>
         </QueryClientProvider>