fix(videocall): apply PR review fixes and add i18n translations

- Replace setState with FutureBuilder in VideoViewWidget
- Remove comments from datasource impl
- Replace GetIt.I with ref.read(provider) in controllers
- Fix state comparison using isTalking getter
- Add hangUp/stopAudio/stopCamera cleanup on dispose
- Remove duplicate SafeArea from IncomingCallOverlay
- Add missedCall error event to enum
- Replace Colors.white70 with theme colorScheme
- Move VIDEOCALL_INTEGRATION.md to apps/mobile_app/docs/
- Gitignore config JSON files with app keys
- Add 28 i18n keys in 6 locales for all videocall strings
- Map error events to specific i18n messages

.gitignore

@@ -24,6 +24,9 @@
 *.iml
 .vscode/
 
+# App config (contains API keys, passed via --dart-define-from-file)
+apps/mobile_app/config/*.json
+
 # macOS
 .DS_Store
 **/.DS_Store