fix(ui): address PR #30 review findings

- ViewportInfo.updateShouldNotify: compare layoutClass only
  (not width/height), avoiding unnecessary rebuilds on every
  resize frame within the same layout class.
- ChatPanel: use BorderDirectional(start:) for RTL support.
- ChatPanel: localize 'Close chat' tooltip via AppL10n.chatCloseAction.
- Inline panel snackbar: localize via AppL10n.chatPanelCollapsedHint.
  New en/zh ARB entries added.
- _saveCurrentDraft(): removed — it was a self-assignment no-op.
  Draft persistence relies on ChatDetailView's didUpdateWidget
  (fires onDraftChanged on target switch) and dispose (fires on
  panel tear-down), both of which already populate _chatDrafts
  correctly without an explicit save call.
- _handleInlineChatViewport layout snackbar: use AppL10n.
- Audio level-meter: switch from callback-count (% 3) to time-based
  gating (std::time::Duration::from_millis(33)), robust to cpal
  buffer-size or sample-rate changes. Remove level_decimation_counter.
- chat_panel_test.dart: add AppL10n.localizationsDelegates so the
  test resolves l10n keys.

Tests: 183 passed, 2 skipped. Dart analyze clean.
cargo test -p chanora_audio --lib: 125 passed.

apps/chanora_flutter/lib/main.dart

@@ -1673,17 +1673,18 @@ class _BetaHomeState extends State<_BetaHome> with WidgetsBindingObserver {
     };
   }
 
-  /// Saves the current draft text for the current inline chat target.
-  void _saveCurrentDraft() {
-    // Drafts are continuously saved via onDraftChanged callback in Task 4.
-    // This method exists as an explicit save point.
-    final target = _inlineChatTarget;
-    if (target == null) return;
-    final key = _draftKeyForTarget(target);
-    if (_chatDrafts.containsKey(key)) {
-      _chatDrafts[key] = _chatDrafts[key] ?? '';
-    }
-  }
+  /// Returns the current draft text for the inline chat target, or null
+  /// if no draft has been stored.
+  ///
+  /// Draft persistence relies on [ChatDetailView]'s own lifecycle:
+  /// `didUpdateWidget` flushes the outgoing target's draft via
+  /// `onDraftChanged` when the parent rebuilds with a new target, and
+  /// `dispose` flushes the final draft when the panel is torn down
+  /// (e.g. on [_closeInlineChat]). Both paths feed
+  /// [_chatDrafts] without requiring an explicit save call from this
+  /// class.
+  String? _currentDraftFor(rust.BridgeMessageTarget target) =>
+      _chatDrafts[_draftKeyForTarget(target)];
 
   Future<void> _onOpenChat({
     rust.BridgeMessageTarget? target,
@@ -1722,8 +1723,9 @@ class _BetaHomeState extends State<_BetaHome> with WidgetsBindingObserver {
         layoutClassFromWidth(MediaQuery.sizeOf(context).width) ==
         LayoutClass.expanded;
     if (isExpanded) {
+      // Outgoing draft is preserved by ChatDetailView.didUpdateWidget,
+      // which fires onDraftChanged with the old target's text on rebuild.
       setState(() {
-        _saveCurrentDraft();
         _chatUnread = 0;
         _chatOpen = true;
         _inlineChatTarget = newTarget;
@@ -1754,8 +1756,10 @@ class _BetaHomeState extends State<_BetaHome> with WidgetsBindingObserver {
   }
 
   void _closeInlineChat() {
+    // Final draft for the dismissed target is preserved by
+    // ChatDetailView.dispose, which fires onDraftChanged when the panel
+    // is removed from the tree on the next rebuild.
     setState(() {
-      _saveCurrentDraft();
       _lastDismissedTarget = _inlineChatTarget;
       _lastDismissedClientName = _inlineChatClientName;
       _chatOpen = false;
@@ -1788,8 +1792,8 @@ class _BetaHomeState extends State<_BetaHome> with WidgetsBindingObserver {
     WidgetsBinding.instance.addPostFrameCallback((_) {
       if (!mounted || _inlineChatTarget == null) return;
       ScaffoldMessenger.of(context).showSnackBar(
-        const SnackBar(
-          content: Text('Tap the chat button to continue your conversation'),
+        SnackBar(
+          content: Text(AppL10n.of(context).chatPanelCollapsedHint),
         ),
       );
     });
@@ -2654,10 +2658,9 @@ class _BetaHomeState extends State<_BetaHome> with WidgetsBindingObserver {
                                 snapshot: _snapshot!,
                                 target: inlineChatTarget,
                                 clientName: _inlineChatClientName,
-                                restoredDraft:
-                                    _chatDrafts[_draftKeyForTarget(
-                                      inlineChatTarget,
-                                    )],
+                                restoredDraft: _currentDraftFor(
+                                  inlineChatTarget,
+                                ),
                                 onDraftChanged: (text) {
                                   final draftKey = _draftKeyForTarget(
                                     inlineChatTarget,