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/l10n/app_en.arb

@@ -46,6 +46,7 @@
   "retryAction": "Retry",
   "chatAction": "Chat",
   "chatCloseAction": "Close chat",
+  "chatPanelCollapsedHint": "Tap the chat button to continue your conversation",
   "chatNewPrivateAction": "New private chat",
   "chatSearchClientsHint": "Search clients...",
   "chatDirectMessageAction": "Private message",

apps/chanora_flutter/lib/l10n/app_zh.arb

@@ -39,6 +39,7 @@
   "retryAction": "重试",
   "chatAction": "聊天",
   "chatCloseAction": "关闭聊天",
+  "chatPanelCollapsedHint": "点击聊天按钮以继续对话",
   "chatNewPrivateAction": "新建私聊",
   "chatSearchClientsHint": "搜索用户...",
   "chatDirectMessageAction": "私聊",

apps/chanora_flutter/lib/l10n/generated/app_localizations.dart

@@ -307,6 +307,12 @@ abstract class AppL10n {
   /// **'Close chat'**
   String get chatCloseAction;
 
+  /// No description provided for @chatPanelCollapsedHint.
+  ///
+  /// In en, this message translates to:
+  /// **'Tap the chat button to continue your conversation'**
+  String get chatPanelCollapsedHint;
+
   /// No description provided for @chatNewPrivateAction.
   ///
   /// In en, this message translates to:

apps/chanora_flutter/lib/l10n/generated/app_localizations_en.dart

@@ -123,6 +123,10 @@ class AppL10nEn extends AppL10n {
   @override
   String get chatCloseAction => 'Close chat';
 
+  @override
+  String get chatPanelCollapsedHint =>
+      'Tap the chat button to continue your conversation';
+
   @override
   String get chatNewPrivateAction => 'New private chat';
 

apps/chanora_flutter/lib/l10n/generated/app_localizations_zh.dart

@@ -120,6 +120,9 @@ class AppL10nZh extends AppL10n {
   @override
   String get chatCloseAction => '关闭聊天';
 
+  @override
+  String get chatPanelCollapsedHint => '点击聊天按钮以继续对话';
+
   @override
   String get chatNewPrivateAction => '新建私聊';