Fix QR scanner crash in release builds and add paste fallback

Summary

Opening Verify → Scan crashes every minified release build (2.0.x, any device). Debug builds aren't minified, so they never show it.

Fixes #878, fixes #970.

Root cause, from the retraced crash and the startup logcat of a release build:

QRCodeAnalyzer.<init>                      (VerificationSheet.kt)
  BarcodeScanning.getClient()              → NullPointerException: ML Kit was never initialized
     because at startup:
     MlKitInitProvider.onCreate
       ComponentDiscovery: Could not instantiate CommonComponentRegistrar,
                           VisionCommonRegistrar, BarcodeRegistrar
         NoSuchMethodException: <init> []  ← R8 full mode removed the no-arg constructors

The fix keeps those constructors. It follows the same pattern as the existing Room rule:

 # app/proguard-rules.pro
+-keepclassmembers class * implements com.google.firebase.components.ComponentRegistrar {
+    <init>();
+}

The Scan tab also gets a fallback and feedback:

 <ScanTabContent>
-  <ScannerView onScan>                 # rejected codes did nothing
+  <ScannerView onScan = handleCode>
+  <TextButton "Paste verification URL"> → handleCode(clipboard text)
+  handleCode: success → toast "Verification requested"
+              failure → inline "Invalid or expired code, or this person isn't connected nearby"

• Paste button: verifies a bitchat://verify link read with another QR app. This helps when the camera is unavailable or camera permission is denied.
• Rejected camera scans: codes that are invalid, expired, or from a peer who isn't connected used to do nothing. They now show the same inline message. It's inline rather than a toast because Android 12+ cuts toasts to two lines.
• Strings: the button and success message reuse the existing, already-translated verify_paste_label and verify_scanned. The new verify_scan_failed is added to every locale, in English where that locale's verify strings are still in English.

Evidence

The tests below used a minified release build signed with the debug key, on an Android 16 emulator unless noted otherwise.

• Before (main):

  • Opening Scan closes the app.
  • Startup logcat shows Could not instantiate …Registrar three times.
  • Removing ImplementationMode.EMBEDDED, as #976 and #974 do, still crashes the same way.

  After:

  • No registrar warnings.
  • The Scan tab shows the camera preview and nothing crashes.
  • The same result on a physical Android 16 phone.

• Camera path: a physical phone scanning an emulator's QR code shows the inline "not connected nearby" message. That's expected, because an emulator can't join a phone's mesh. It shows the camera and ML Kit decoding work end to end.

• Successful verification: two emulators meshed over the emulator's virtual Bluetooth.

  • Pasting emulator A's verify link on emulator B shows "Verification requested".
  • B then lists A with the verified badge and the message "Verified ⟨A⟩".

• Paste with an empty clipboard: shows the inline error and doesn't crash.

• CI tasks: testDebugUnitTest passes (638 tests, 0 failures). lintDebug reports no new issues in the changed files.

Mesh Lab wasn't run: no transport, routing, protocol or crypto code changed.

Merge Danger

Door: two-way

Blast Radius: small

• The keep rule only matches classes implementing ComponentRegistrar, which here are ML Kit's three registrars.
• The UI change is limited to the Scan tab.
• Related PRs #976, #900 and #899 change the CameraX viewfinder setup but don't address this crash. For #976's change, I checked this on the release build.

🤖 Generated with Claude Code