ui: transparent compact AppBar (64dp → 44dp)

- AppBar background transparent, merges with scaffold for seamless look
- toolbarHeight reduced from 64dp to 44dp (~20dp screen space saved)
- scrolledUnderElevation: 0 prevents Material 3 shadow on scroll
- Icons 24→20px with VisualDensity.compact for tighter action buttons
- Title fontSize 16 w600, less visual weight
- Both dark and light themes updated consistently

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

it0_app/lib/core/theme/app_theme.dart

@@ -13,8 +13,10 @@ class AppTheme {
     ),
     scaffoldBackgroundColor: AppColors.background,
     appBarTheme: const AppBarTheme(
-      backgroundColor: AppColors.surface,
+      backgroundColor: Colors.transparent,
       elevation: 0,
+      scrolledUnderElevation: 0,
+      toolbarHeight: 44,
     ),
     cardTheme: CardThemeData(
       color: AppColors.surface,
@@ -66,8 +68,10 @@ class AppTheme {
     ),
     scaffoldBackgroundColor: const Color(0xFFF1F5F9),
     appBarTheme: const AppBarTheme(
-      backgroundColor: Colors.white,
+      backgroundColor: Colors.transparent,
       elevation: 0,
+      scrolledUnderElevation: 0,
+      toolbarHeight: 44,
       foregroundColor: Color(0xFF1E293B),
     ),
     cardTheme: CardThemeData(

it0_app/lib/features/chat/presentation/pages/chat_page.dart

@@ -476,27 +476,28 @@ class _ChatPageState extends ConsumerState<ChatPage> {
     return Scaffold(
       drawer: const ConversationDrawer(),
       appBar: AppBar(
-        title: const Text('iAgent'),
+        title: const Text('iAgent', style: TextStyle(fontSize: 16, fontWeight: FontWeight.w600)),
         actions: [
-          // New chat button (always visible)
           IconButton(
-            icon: const Icon(Icons.edit_outlined),
+            icon: const Icon(Icons.edit_outlined, size: 20),
             tooltip: '新对话',
+            visualDensity: VisualDensity.compact,
             onPressed: () => ref.read(chatProvider.notifier).startNewChat(),
           ),
-          // Stop button during streaming
           if (chatState.isStreaming)
             IconButton(
-              icon: const Icon(Icons.stop_circle_outlined),
+              icon: const Icon(Icons.stop_circle_outlined, size: 20),
               tooltip: '停止',
+              visualDensity: VisualDensity.compact,
               onPressed: () => ref.read(chatProvider.notifier).cancelCurrentTask(),
             ),
-          // Voice call button
           IconButton(
-            icon: const Icon(Icons.call),
+            icon: const Icon(Icons.call, size: 20),
             tooltip: '语音通话',
+            visualDensity: VisualDensity.compact,
             onPressed: _openVoiceCall,
           ),
+          const SizedBox(width: 4),
         ],
       ),
       body: SafeArea(