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>

renderer/index.html

@@ -49,6 +49,7 @@
       <div class="fleet-head">
         <h2>Dashboard</h2>
         <div class="row">
+          <span id="dash-cache" class="dash-cache" title=""></span>
           <label class="inline dash-toggle">
             <input id="dash-extra" type="checkbox" />
             Detailed stats (Rx / FEC scan)