feat: independent canvas per tab

- Added showCanvas to AIAssistTab interface in store
- Each tab now has its own canvas visibility state
- Canvas state syncs when switching tabs
- Closing canvas saves state to tab
- Clearing history resets canvas state
- Streaming updates save showCanvas: true to the request's tab
- Complete tab isolation: history, agent, preview, AND canvas

lib/store.ts

@@ -6,7 +6,8 @@ interface AIAssistTab {
   title: string;
   history: AIAssistMessage[];
   currentAgent: string;
-  previewData?: any | null; // PreviewData type from AIAssist
+  previewData?: any | null;
+  showCanvas?: boolean;
 }
 
 interface AppState {