Dashboard: rebuild into an operator cockpit (0.38.0)

Replace the analytics-style dashboard (stack-health funnel, uptime %,
operations/day grid, AI pill) with an attention-driven fleet cockpit:

- New /api/dashboard/fleet endpoint: server-side fan-out across the local
  host and every agent into one payload — a prioritized "needs attention"
  list, headline KPIs, an honest stack-status breakdown and a per-host
  resource rollup. Each agent uses its own DB session so the fan-out is
  concurrency-safe; failures degrade to "offline" instead of stalling.
- New frontend: AttentionStrip, FleetKpiRow, StackStatusBar and
  HostResourceTable; Dashboard.tsx rewritten around them.
- Remove the funnel/summary endpoints, the uptime sampler loop and the
  ops-activity machinery; delete the now-unused chart components.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -36,7 +36,7 @@ from routers import (
     volumes,
     ws,
 )
-from services import dashboard_service, schedule_service, template_service, update_service
+from services import schedule_service, template_service, update_service
 
 logging.basicConfig(level=logging.INFO)
 logger = logging.getLogger("stackpilot")
@@ -59,12 +59,10 @@ async def lifespan(app: FastAPI):
         logger.warning("Legacy template migration failed: %s", exc)
     update_task = asyncio.create_task(update_service.background_loop())
     schedule_task = asyncio.create_task(schedule_service.scheduler_loop())
-    uptime_task = asyncio.create_task(dashboard_service.uptime_sampler_loop())
     logger.info("StackPilot backend ready on port %s", settings.PORT)
     yield
     update_task.cancel()
     schedule_task.cancel()
-    uptime_task.cancel()
 
 
 app = FastAPI(title="StackPilot", version=APP_VERSION, lifespan=lifespan)