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/test/widgets/chat_panel_test.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import 'package:chanora_flutter/l10n/generated/app_localizations.dart';
 import 'package:chanora_flutter/src/rust/api.dart' as rust;
 import 'package:chanora_flutter/widgets/chat_panel.dart';
 import 'package:chanora_flutter/widgets/chat_views.dart';
@@ -55,6 +56,8 @@ void main() {
 
     await tester.pumpWidget(
       MaterialApp(
+        localizationsDelegates: AppL10n.localizationsDelegates,
+        supportedLocales: AppL10n.supportedLocales,
         home: Scaffold(
           body: ChatPanel(
             messages: messages,
@@ -87,6 +90,8 @@ void main() {
       required String? restoredDraft,
     }) {
       return MaterialApp(
+        localizationsDelegates: AppL10n.localizationsDelegates,
+        supportedLocales: AppL10n.supportedLocales,
         home: Scaffold(
           body: ChatDetailView(
             messages: messages,