Optimize chat rendering & add web session activity tracking (#214)

* feat: add chat virtualization and turn grouping infrastructure

Implement VirtualMessageList with virtualization and overscan for smooth scrolling
Refactor MessageList to render only visible messages and pass scroll refs
Introduce TurnGroupingContext and provider to stabilize per-turn rendering and reduce re-renders

* feat: add web server session activity endpoint for visibility restore

Add /api/session-activity endpoint to expose tracked session activity
Use web server activity first when restoring visibility, with fallback to global status
Update server SSE to set session phase on activity events

* feat(chat): split TurnGroupingContext into UI/Streaming contexts

Add separate UI state and streaming contexts to reduce re-renders.
Skip animations for items visible in collapsed view when expanding.
Track shown parts in collapsed mode to fade in progressively

* feat(ui): cache turn groups with expand state and guard web activity

Add isExpanded to the turn cache key to trigger updates correctly
Expose isGroupExpanded from UI state so groups reflect expand/collapse
Limit web server session activity fetch to web runtime only

packages/ui/src/lib/opencode/client.ts

@@ -815,6 +815,38 @@ class OpencodeService {
     return this.getSessionStatusForDirectory(null);
   }
 
+  /**
+   * Get session activity from web server's in-memory tracking.
+   * This is more reliable than getGlobalSessionStatus on visibility restore
+   * because the web server tracks activity even when UI is not listening to SSE.
+   */
+  async getWebServerSessionActivity(): Promise<
+    Record<string, { type: string }> | null
+  > {
+    try {
+      // Web server endpoint - use relative path that works with both dev and prod
+      const response = await fetch('/api/session-activity', {
+        method: 'GET',
+        headers: {
+          Accept: 'application/json',
+        },
+      });
+
+      if (!response.ok) {
+        return null;
+      }
+
+      const data = await response.json().catch(() => null);
+      if (!data || typeof data !== 'object') {
+        return null;
+      }
+
+      return data as Record<string, { type: string }>;
+    } catch {
+      return null;
+    }
+  }
+
   // Tools
   async listToolIds(options?: { directory?: string | null }): Promise<string[]> {
     try {