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/widgets/chat_panel.dart

@@ -3,6 +3,7 @@
 import 'package:flutter/material.dart';
 
 import '../design/breakpoints.dart';
+import '../l10n/generated/app_localizations.dart';
 import '../services/snapshot_state_mapper.dart';
 import '../services/ts3_server_link.dart';
 import '../src/rust/api.dart' as rust;
@@ -51,14 +52,15 @@ class ChatPanel extends StatelessWidget {
   Widget build(BuildContext context) {
     final currentChannelId = ownClientSnapshotState(snapshot)?.channelId;
     final channelName = snapshotChannelName(snapshot, currentChannelId);
+    final l10n = AppL10n.of(context);
 
     return SizedBox(
       width: ChanoraBreakpoints.chatPanelWidth,
       child: DecoratedBox(
         decoration: BoxDecoration(
           color: Theme.of(context).colorScheme.surface,
-          border: Border(
-            left: BorderSide(
+          border: BorderDirectional(
+            start: BorderSide(
               color: Theme.of(context).colorScheme.outlineVariant,
             ),
           ),
@@ -75,7 +77,7 @@ class ChatPanel extends StatelessWidget {
           onDraftChanged: onDraftChanged,
           messageMaxWidth: 500,
           headerTrailing: IconButton(
-            tooltip: 'Close chat',
+            tooltip: l10n.chatCloseAction,
             icon: const Icon(Icons.close),
             onPressed: onClose,
           ),