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>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.37.7",
+  "version": "0.38.0",
   "type": "module",
   "scripts": {
     "dev": "vite",