Mobile: QR-code verification

When both sides support it, a verification offers QR codes first: this
device shows its code (in a white quiet zone, whatever the theme) and can
scan the other's with the camera (mobile_scanner; only codes that start
with MATRIX count). The side whose code was scanned confirms the other
saw it succeed. Compare emoji stays available, and is used directly when
either side has no QR support.

Adds the CAMERA permission (camera hardware not required).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/mobile/android/app/src/main/AndroidManifest.xml

@@ -3,6 +3,9 @@
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
     <uses-permission android:name="android.permission.VIBRATE" />
+    <!-- Scanning a QR code to verify a session or a person. -->
+    <uses-permission android:name="android.permission.CAMERA" />
+    <uses-feature android:name="android.hardware.camera" android:required="false" />
 
     <application
         android:label="Warp"