feat: implement dashboard widgets functionality

Backend:
- Add dashboard API router with widget endpoints
- Create dashboard schemas for widget data
- Add dashboard tests

Frontend:
- Enhance Dashboard page with widget components
- Add dashboard service for API calls
- Create reusable dashboard components

OpenSpec:
- Archive add-dashboard-widgets change
- Add dashboard capability specs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/main.py

@@ -37,6 +37,7 @@ from app.api.health import router as health_router
 from app.api.custom_fields import router as custom_fields_router
 from app.api.task_dependencies import router as task_dependencies_router
 from app.api.admin import encryption_keys as admin_encryption_keys_router
+from app.api.dashboard import router as dashboard_router
 from app.core.config import settings
 
 app = FastAPI(
@@ -82,6 +83,7 @@ app.include_router(health_router)
 app.include_router(custom_fields_router)
 app.include_router(task_dependencies_router)
 app.include_router(admin_encryption_keys_router.router)
+app.include_router(dashboard_router, prefix="/api/dashboard", tags=["Dashboard"])
 
 
 @app.get("/health")