feat: floating pill input bar + auto-scroll on history load

Input area redesign (ChatGPT/Claude App style):
- Replace fixed bottom bar with floating pill overlay using Stack+Positioned
- Semi-transparent background (surface 92% opacity) with rounded corners (28px)
- Drop shadow for depth separation from content
- Remove inner TextField border (InputBorder.none) for cleaner look
- ListView bottom padding increased to 80px to leave room under the pill
- Input pill floats 12px from edges, 8px from bottom

History scroll fix:
- Add jump parameter to _scrollToBottom() for instant positioning
- When loading conversation history (empty→many messages), use jumpTo
  instead of animateTo to avoid incomplete scroll on large message lists
- Double-frame jumpTo ensures layout settles before final scroll position

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

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

@@ -53,11 +53,23 @@ class _ChatPageState extends ConsumerState<ChatPage> {
     _scrollToBottom();
   }
 
-  void _scrollToBottom() {
+  void _scrollToBottom({bool jump = false}) {
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!_scrollController.hasClients) return;
+      final target = _scrollController.position.maxScrollExtent + 80;
+      if (jump) {
+        _scrollController.jumpTo(target);
+        // Second frame: layout may still be settling for large message lists
         WidgetsBinding.instance.addPostFrameCallback((_) {
           if (_scrollController.hasClients) {
-        _scrollController.animateTo(
+            _scrollController.jumpTo(
               _scrollController.position.maxScrollExtent + 80,
+            );
+          }
+        });
+      } else {
+        _scrollController.animateTo(
+          target,
           duration: const Duration(milliseconds: 300),
           curve: Curves.easeOut,
         );
@@ -454,7 +466,12 @@ class _ChatPageState extends ConsumerState<ChatPage> {
     final chatState = ref.watch(chatProvider);
 
     // Auto-scroll when messages change
-    ref.listen(chatProvider, (_, __) => _scrollToBottom());
+    ref.listen(chatProvider, (prev, next) {
+      // Jump (no animation) when loading a conversation history
+      final wasEmpty = prev?.messages.isEmpty ?? true;
+      final nowHasMany = next.messages.length > 1;
+      _scrollToBottom(jump: wasEmpty && nowHasMany);
+    });
 
     return Scaffold(
       drawer: const ConversationDrawer(),
@@ -510,20 +527,20 @@ class _ChatPageState extends ConsumerState<ChatPage> {
               ),
             ),
 
-          // Timeline message list
+          // Timeline message list + floating input
           Expanded(
-            child: chatState.messages.isEmpty
+            child: Stack(
+              children: [
+                // Messages fill the entire area
+                chatState.messages.isEmpty
                     ? _buildEmptyState()
                     : ListView.builder(
                         controller: _scrollController,
-                    padding: const EdgeInsets.fromLTRB(12, 8, 12, 8),
+                        // Bottom padding leaves room for the floating input pill
-                    // Extra item for the "working" placeholder when streaming
+                        padding: const EdgeInsets.fromLTRB(12, 8, 12, 80),
-                    // and the last message is still a user message (no agent
-                    // response has arrived yet).
                         itemCount: chatState.messages.length +
                             (_needsWorkingNode(chatState) ? 1 : 0),
                         itemBuilder: (context, index) {
-                      // Render the virtual "working" node at the end
                           if (index == chatState.messages.length &&
                               _needsWorkingNode(chatState)) {
                             return TimelineEventNode(
@@ -545,10 +562,16 @@ class _ChatPageState extends ConsumerState<ChatPage> {
                           );
                         },
                       ),
+                // Floating input pill at bottom
+                Positioned(
+                  left: 12,
+                  right: 12,
+                  bottom: 8,
+                  child: _buildInputArea(chatState),
+                ),
+              ],
+            ),
           ),
-
-          // Input row
-          _buildInputArea(chatState),
         ],
       ),
       ),
@@ -592,61 +615,83 @@ class _ChatPageState extends ConsumerState<ChatPage> {
     final isStreaming = chatState.isStreaming && !isAwaitingApproval;
 
     return Container(
-      padding: const EdgeInsets.all(8),
       decoration: BoxDecoration(
-        color: AppColors.surface,
+        color: AppColors.surface.withOpacity(0.92),
-        border: Border(top: BorderSide(color: AppColors.surfaceLight.withOpacity(0.5))),
+        borderRadius: BorderRadius.circular(28),
+        border: Border.all(color: AppColors.surfaceLight.withOpacity(0.6)),
+        boxShadow: [
+          BoxShadow(
+            color: Colors.black.withOpacity(0.25),
+            blurRadius: 12,
+            offset: const Offset(0, 2),
+          ),
+        ],
       ),
       child: Column(
         mainAxisSize: MainAxisSize.min,
         children: [
-          if (_pendingAttachments.isNotEmpty) _buildAttachmentPreview(),
+          if (_pendingAttachments.isNotEmpty)
+            Padding(
+              padding: const EdgeInsets.fromLTRB(12, 8, 12, 0),
+              child: _buildAttachmentPreview(),
+            ),
           Row(
             children: [
               if (!isStreaming)
-                IconButton(
+                Padding(
-                  icon: const Icon(Icons.add_circle_outline),
+                  padding: const EdgeInsets.only(left: 4),
+                  child: IconButton(
+                    icon: const Icon(Icons.add_circle_outline, size: 22),
                     tooltip: '添加图片',
                     onPressed: isAwaitingApproval ? null : _showAttachmentOptions,
                   ),
+                ),
               Expanded(
                 child: TextField(
                   controller: _messageController,
                   decoration: InputDecoration(
                     hintText: isStreaming ? '追加指令...' : '输入指令...',
                     hintStyle: TextStyle(color: AppColors.textMuted),
-                    border: const OutlineInputBorder(
+                    border: InputBorder.none,
-                      borderRadius: BorderRadius.all(Radius.circular(24)),
+                    contentPadding: EdgeInsets.only(
+                      left: isStreaming ? 16 : 4,
+                      right: 4,
+                      top: 12,
+                      bottom: 12,
                     ),
-                    contentPadding: const EdgeInsets.symmetric(horizontal: 16, vertical: 10),
                   ),
                   textInputAction: TextInputAction.send,
                   onSubmitted: (_) => isStreaming ? _inject() : _send(),
                   enabled: !isAwaitingApproval,
                 ),
               ),
-              const SizedBox(width: 8),
               if (isStreaming)
                 Row(
                   mainAxisSize: MainAxisSize.min,
                   children: [
                     IconButton(
-                      icon: const Icon(Icons.send, color: AppColors.info),
+                      icon: const Icon(Icons.send, color: AppColors.info, size: 20),
                       tooltip: '追加指令',
                       onPressed: _inject,
                     ),
-                    IconButton(
+                    Padding(
-                      icon: const Icon(Icons.stop_circle_outlined, color: AppColors.error),
+                      padding: const EdgeInsets.only(right: 4),
+                      child: IconButton(
+                        icon: const Icon(Icons.stop_circle_outlined, color: AppColors.error, size: 20),
                         tooltip: '停止',
                         onPressed: () => ref.read(chatProvider.notifier).cancelCurrentTask(),
                       ),
+                    ),
                   ],
                 )
               else
-                IconButton(
+                Padding(
-                  icon: const Icon(Icons.send),
+                  padding: const EdgeInsets.only(right: 4),
+                  child: IconButton(
+                    icon: const Icon(Icons.send, size: 20),
                     onPressed: isAwaitingApproval ? null : _send,
                   ),
+                ),
             ],
           ),
         ],