feat: mobile Nostr identity — Amber NIP-55 + nsec fallback (Fixes #29)

- Add NostrIdentityContext with SecureStore-backed nsec storage
  and pure-JS bech32/secp256k1 for nsec→npub derivation; private key
  never enters React state or logs
- Android: NIP-55 Amber deep-link integration (get_public_key +
  sign_event) with install-prompt fallback to Play Store when Amber
  is absent; Android queries manifest entry for com.greenart7c3.nostrsigner
- iOS/both: manual nsec entry stored exclusively in expo-secure-store
- Settings tab (gear icon) added to both NativeTabLayout and
  ClassicTabLayout showing: connected npub (truncated), signing method
  badge, Disconnect button (with confirmation + SecureStore wipe)
- Root layout wrapped with NostrIdentityProvider
- app.json: add expo-secure-store plugin + Android intentFilters for
  mobile://amber-callback deep-link return path

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

artifacts/mobile/package.json

@@ -25,6 +25,7 @@
     "babel-plugin-react-compiler": "^19.0.0-beta-e993439-20250117",
     "expo": "~54.0.27",
     "expo-blur": "~15.0.8",
+    "expo-secure-store": "~14.0.1",
     "expo-constants": "~18.0.11",
     "expo-font": "~14.0.10",
     "expo-glass-effect": "~0.1.4",