dashboard: live cache-age indicator

The web server's dashboardStats now reports how stale its shared cache
snapshot is — data.cache = { enabled, ttlMs, ageMs }, using the age of the
oldest dataset feeding the view (OLT-STATE, plus ONU-STATE under detailed
scan). cache.js gains ageOf(key).

The dashboard renders a live-ticking badge from it (renderCacheBadge):
  - within TTL  -> "◷ cached Ns ago" (info)
  - past TTL    -> same, warn-coloured (next load will refetch)
  - no cache    -> "● live" (Electron app, or PFW_CACHE_TTL_SECONDS=0)
The Dashboard Refresh button now sends fresh:true to bypass the cache and
reset the age; the detailed-stats toggle and auto-loads use the cache.

Shared renderer change, so the Electron app shows "live" (it sends no cache
field). Verified offscreen over HTTP: cached / stale / live states render
with the right text + colour.

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

CLAUDE.md

@@ -803,6 +803,11 @@ Consequences for editing:
   read-modify-write fetches and the FEC pre-flight stay **uncached** (must be
   live). Writes call `invalidate(session, [...])` for the affected datasets.
   This cache is web-only; the single-user Electron app doesn't need it.
+  `dashboardStats` attaches `data.cache = { enabled, ttlMs, ageMs }` (age of
+  the oldest dataset feeding the view); the dashboard renders a live-ticking
+  age badge from it (`renderCacheBadge`), and the Dashboard **Refresh** button
+  sends `fresh:true` to bypass the cache. The Electron app sends no cache
+  field, so the badge shows "live".
 - **Deploy** lives in `web/deploy/` (systemd unit + env template +
   `update.sh`); Debian-13-LXC clone-to-run steps and the Nginx Proxy Manager
   setup are in `web/README.md`. Note streams send `X-Accel-Buffering: no` so