feat: collapsible thinking node in chat timeline

Thinking content auto-expands while streaming, auto-collapses when done.
User can toggle with "Thinking ∨" button, matching Claude Code VSCode UX.

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

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

@@ -89,14 +89,9 @@ class _ChatPageState extends ConsumerState<ChatPage> {
           label: '思考中...',
           isFirst: isFirst,
           isLast: isLast,
-          content: StreamTextWidget(
+          content: _CollapsibleThinking(
             text: message.content,
             isStreaming: isStreamingNow,
-            style: const TextStyle(
-              color: AppColors.textSecondary,
-              fontSize: 13,
-              fontStyle: FontStyle.italic,
-            ),
           ),
         );
 
@@ -378,6 +373,108 @@ class _ChatPageState extends ConsumerState<ChatPage> {
 // Standing order content (embedded in timeline node)
 // ---------------------------------------------------------------------------
 
+// ---------------------------------------------------------------------------
+// Collapsible thinking content – expanded while streaming, collapsed when done
+// ---------------------------------------------------------------------------
+
+class _CollapsibleThinking extends StatefulWidget {
+  final String text;
+  final bool isStreaming;
+
+  const _CollapsibleThinking({
+    required this.text,
+    required this.isStreaming,
+  });
+
+  @override
+  State<_CollapsibleThinking> createState() => _CollapsibleThinkingState();
+}
+
+class _CollapsibleThinkingState extends State<_CollapsibleThinking> {
+  bool _expanded = true;
+
+  @override
+  void didUpdateWidget(covariant _CollapsibleThinking oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    // Auto-collapse when streaming ends
+    if (oldWidget.isStreaming && !widget.isStreaming) {
+      setState(() => _expanded = false);
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    // While streaming, always show expanded content
+    if (widget.isStreaming) {
+      return StreamTextWidget(
+        text: widget.text,
+        isStreaming: true,
+        style: const TextStyle(
+          color: AppColors.textSecondary,
+          fontSize: 13,
+          fontStyle: FontStyle.italic,
+        ),
+      );
+    }
+
+    // Completed – show collapsible toggle
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        GestureDetector(
+          onTap: () => setState(() => _expanded = !_expanded),
+          child: Row(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              Text(
+                'Thinking',
+                style: TextStyle(
+                  color: AppColors.textMuted,
+                  fontSize: 12,
+                  fontWeight: FontWeight.w500,
+                ),
+              ),
+              const SizedBox(width: 4),
+              AnimatedRotation(
+                turns: _expanded ? 0.5 : 0.0,
+                duration: const Duration(milliseconds: 200),
+                child: Icon(
+                  Icons.expand_more,
+                  size: 16,
+                  color: AppColors.textMuted,
+                ),
+              ),
+            ],
+          ),
+        ),
+        AnimatedCrossFade(
+          firstChild: Padding(
+            padding: const EdgeInsets.only(top: 6),
+            child: StreamTextWidget(
+              text: widget.text,
+              isStreaming: false,
+              style: const TextStyle(
+                color: AppColors.textSecondary,
+                fontSize: 13,
+                fontStyle: FontStyle.italic,
+              ),
+            ),
+          ),
+          secondChild: const SizedBox.shrink(),
+          crossFadeState: _expanded
+              ? CrossFadeState.showFirst
+              : CrossFadeState.showSecond,
+          duration: const Duration(milliseconds: 200),
+        ),
+      ],
+    );
+  }
+}
+
+// ---------------------------------------------------------------------------
+// Standing order content (embedded in timeline node)
+// ---------------------------------------------------------------------------
+
 class _StandingOrderContent extends StatelessWidget {
   final Map<String, dynamic> draft;
   final VoidCallback onConfirm;