Reduce Linux setup ambiguity and surface desktop input/message failures honestly

Clarify ONNX Runtime guidance with direct-open install hints, restore desktop WebRTC VAD visibility, map mouse side buttons through focused PTT capture/runtime paths, and wait for server acks before showing chat sends as successful.

Constraint: Linux release UX must stay functional when ONNX Runtime is optional and GNOME portal availability varies
Rejected: Keep desktop VAD locked to Silero only | misleads users when ONNX Runtime is skipped
Confidence: medium
Scope-risk: moderate
Directive: Preserve the protocol send-ack wait path for chat so UI success always tracks real server acceptance
Tested: flutter analyze lib/main.dart lib/widgets/chat_views.dart lib/widgets/input_dialogs.dart lib/widgets/startup_dependency_screen.dart; flutter test test/widgets/input_dialogs_test.dart test/widgets/chat_views_test.dart test/services/startup_dependency_check_test.dart test/widgets/startup_dependency_screen_test.dart test/widgets/voice_settings_controls_test.dart test/widgets/audio_processing_config_state_test.dart; cargo test -p chanora_protocol --lib; cargo test -p chanora_audio ptt_backends --lib
Not-tested: Live manual GNOME portal rebind/global PTT on a real desktop session; observer-bot chat against a live server after the sender-name fallback change

apps/chanora_flutter/lib/widgets/voice_settings.dart

@@ -299,6 +299,14 @@ class _VoiceSettingsDialogState extends State<VoiceSettingsDialog> {
                     setState(() => _audioProcessing.vadBackend = s.first),
               ),
               const SizedBox(height: 8),
+              if (_isDesktopSileroVadHost)
+                Text(
+                  'Silero gives the best quality when ONNX Runtime is installed. WebRTC works without ONNX Runtime and is the safer fallback if Linux setup is incomplete.',
+                  style: theme.textTheme.bodySmall?.copyWith(
+                    color: theme.colorScheme.onSurfaceVariant,
+                  ),
+                ),
+              if (_isDesktopSileroVadHost) const SizedBox(height: 8),
 
               // ── PTT capability badge ────────────────────────────────
               if (_mode == rust.BridgeTransmitMode.ptt &&