feat(nip46): register nostrconnect:// deep link to the signer

Add a VIEW/BROWSABLE intent filter for the `nostrconnect` scheme on
MainActivity so a website's "Connect with Amethyst" link (or any app
firing a nostrconnect:// URI) hands off into the same pairing flow as a
scanned QR. The incoming intent.data already flows through uriToRoute,
which now maps a nostrconnect:// offer to Route.Nip46Signer(connectUri);
both the cold-start and onNewIntent paths navigate there and pair on open.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015FHr2mu5SiHwYNR7evYUuF

amethyst/src/main/AndroidManifest.xml

@@ -151,6 +151,14 @@
                 <data android:scheme="amethyst+walletconnect" />
             </intent-filter>
 
+            <!-- NIP-46: an app's `nostrconnect://` offer opens the signer screen, which pairs it. -->
+            <intent-filter android:label="Amethyst">
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data android:scheme="nostrconnect" />
+            </intent-filter>
+
             <intent-filter android:label="New Post">
                 <action android:name="android.intent.action.SEND" />
                 <category android:name="android.intent.category.DEFAULT" />