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.json

@@ -20,7 +20,18 @@
       "adaptiveIcon": {
         "foregroundImage": "./assets/images/icon.png",
         "backgroundColor": "#0A0A12"
-      }
+      },
+      "intentFilters": [
+        {
+          "action": "VIEW",
+          "autoVerify": true,
+          "data": [{ "scheme": "mobile", "host": "amber-callback" }],
+          "category": ["BROWSABLE", "DEFAULT"]
+        }
+      ],
+      "queries": [
+        { "package": "com.greenart7c3.nostrsigner" }
+      ]
     },
     "web": {
       "favicon": "./assets/images/icon.png",
@@ -34,6 +45,7 @@
         }
       ],
       "expo-font",
+      "expo-secure-store",
       "expo-web-browser"
     ],
     "extra": {