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/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 { NostrIdentityProvider } from "@/context/NostrIdentityContext";
 import { ONBOARDING_COMPLETED_KEY } from "@/constants/storage-keys";
 
 SplashScreen.preventAutoHideAsync();
@@ -76,9 +77,11 @@ export default function RootLayout() {
         <QueryClientProvider client={queryClient}>
           <GestureHandlerRootView style={{ flex: 1 }}>
             <KeyboardProvider>
-              <TimmyProvider>
-                <RootLayoutNav />
-              </TimmyProvider>
+              <NostrIdentityProvider>
+                <TimmyProvider>
+                  <RootLayoutNav />
+                </TimmyProvider>
+              </NostrIdentityProvider>
             </KeyboardProvider>
           </GestureHandlerRootView>
         </QueryClientProvider>