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/input_dialogs.dart

@@ -4,6 +4,9 @@ import 'package:flutter/services.dart';
 import '../l10n/generated/app_localizations.dart';
 import '../src/rust/api.dart' as rust;
 
+const int pttMouseBackButtonBitmask = 0x08;
+const int pttMouseForwardButtonBitmask = 0x10;
+
 /// Translate a [LogicalKeyboardKey] into the platform-neutral label
 /// stored by the PTT binding flow.
 String? pttDisplayLabelForKey(LogicalKeyboardKey k) {
@@ -46,6 +49,27 @@ String? pttDisplayLabelForKey(LogicalKeyboardKey k) {
   return fallback;
 }
 
+String? pttMouseSideButtonPlatformKeyForLogicalKey(LogicalKeyboardKey key) {
+  return switch (key) {
+    LogicalKeyboardKey.browserBack ||
+    LogicalKeyboardKey.goBack => 'mouse-side-button:$pttMouseBackButtonBitmask',
+    LogicalKeyboardKey.browserForward =>
+      'mouse-side-button:$pttMouseForwardButtonBitmask',
+    _ => null,
+  };
+}
+
+String? pttMouseSideButtonPlatformKeyForButtons(int buttons) {
+  if ((buttons & pttMouseBackButtonBitmask) == pttMouseBackButtonBitmask) {
+    return 'mouse-side-button:$pttMouseBackButtonBitmask';
+  }
+  if ((buttons & pttMouseForwardButtonBitmask) ==
+      pttMouseForwardButtonBitmask) {
+    return 'mouse-side-button:$pttMouseForwardButtonBitmask';
+  }
+  return null;
+}
+
 /// Result of a successful PTT binding capture.
 class CapturedBinding {
   const CapturedBinding({required this.inputClass, required this.platformKey});
@@ -181,6 +205,16 @@ class _PttBindingCaptureDialogState extends State<PttBindingCaptureDialog> {
 
   KeyEventResult _onKeyEvent(FocusNode node, KeyEvent event) {
     if (event is! KeyDownEvent) return KeyEventResult.ignored;
+    final mouseSideButton = pttMouseSideButtonPlatformKeyForLogicalKey(
+      event.logicalKey,
+    );
+    if (mouseSideButton != null) {
+      setState(() {
+        _captured = mouseSideButton;
+        _capturedClass = rust.BridgePttInputClass.mouseSideButton;
+      });
+      return KeyEventResult.handled;
+    }
     final label = pttDisplayLabelForKey(event.logicalKey);
     if (label == null) return KeyEventResult.ignored;
     setState(() {
@@ -190,9 +224,9 @@ class _PttBindingCaptureDialogState extends State<PttBindingCaptureDialog> {
     return KeyEventResult.handled;
   }
 
-  void _captureMouseSideButton(int button) {
+  void _captureMouseSideButton(String platformKey) {
     setState(() {
-      _captured = 'mouse-side-button:$button';
+      _captured = platformKey;
       _capturedClass = rust.BridgePttInputClass.mouseSideButton;
     });
   }
@@ -212,10 +246,11 @@ class _PttBindingCaptureDialogState extends State<PttBindingCaptureDialog> {
           child: Listener(
             behavior: HitTestBehavior.opaque,
             onPointerDown: (e) {
-              const int back = 0x08;
-              const int forward = 0x10;
-              if (e.buttons == back || e.buttons == forward) {
-                _captureMouseSideButton(e.buttons);
+              final platformKey = pttMouseSideButtonPlatformKeyForButtons(
+                e.buttons,
+              );
+              if (platformKey != null) {
+                _captureMouseSideButton(platformKey);
               }
             },
             child: Column(