0.31.1: make dashboard metrics honest

- Container card now compares compose-only counts across hosts: agents
  report compose_running in /agent/system (pre-0.31.1 agents fall back to
  the all-containers number); card retitled, ResourceBar stat labelled
  'Containers (all)'.
- Uptime is sampled every 5 min (background loop + opportunistic on read)
  and charted as daily averages instead of a once-a-day snapshot; no
  sample is written when no compose containers exist (was: fake 100%).
  Legacy daily entries in uptime.jsonl still count; file pruned at startup.
- Funnel stage 'monitored' is now per-stack and real: stacks with an
  enabled local auto-update policy (was: global webhook-exists toggle).
  Frontend label renamed to 'Auto-managed'.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.31.0",
+  "version": "0.31.1",
   "type": "module",
   "scripts": {
     "dev": "vite",