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/constants/colors.ts

@@ -29,6 +29,10 @@ export const Colors = {
     working: "#F59E0B",
     idle: "#6B7280",
     micActive: "#EF4444",
+    field: "#1A1A2E",
+    textInverted: "#0A0A12",
+    destructive: "#EF4444",
+    link: "#A78BFA",
   },
 } as const;
 

artifacts/mobile/constants/storage-keys.ts

@@ -1 +1,2 @@
 export const ONBOARDING_COMPLETED_KEY = "app.onboarding_completed";
+export const SERVER_URL_KEY = "settings.server_url";