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/(tabs)/_layout.tsx

@@ -25,6 +25,10 @@ function NativeTabLayout() {
         <Icon sf={{ default: "list.bullet", selected: "list.bullet.circle.fill" }} />
         <Label>Feed</Label>
       </NativeTabs.Trigger>
+      <NativeTabs.Trigger name="settings">
+        <Icon sf={{ default: "gearshape", selected: "gearshape.fill" }} />
+        <Label>Settings</Label>
+      </NativeTabs.Trigger>
     </NativeTabs>
   );
 }
@@ -100,6 +104,18 @@ function ClassicTabLayout() {
             ),
         }}
       />
+      <Tabs.Screen
+        name="settings"
+        options={{
+          title: "Settings",
+          tabBarIcon: ({ color, size }) =>
+            isIOS ? (
+              <SymbolView name="gearshape" tintColor={color} size={size} />
+            ) : (
+              <Feather name="settings" size={size} color={color} />
+            ),
+        }}
+      />
     </Tabs>
   );
 }