Server: lifetime stats counters in a collapsed web UI section

New FrameStats (first_seen, device_wakes, photos_displayed,
photos_removed, battery_reports, recharge_cycles, ota_updates_applied,
config_saves), persisted alongside everything else in config.json.
Incremented at the existing route/photo_queue.py call sites that already
own each event -- no new instrumentation plumbing, no behavior depends
on these, purely informational. GET /api/stats serves them; the web UI
renders them into a native <details> "Stats" card (collapsed by default,
no JS needed for the expand/collapse), fetched once on page load like
the battery-history chart.

Verified: TestClient run through /frame/config (wakes + first_seen +
OTA-applied detection), /frame/battery (reports + recharge detection),
/api/config (saves); direct photo_queue.py unit checks for
advance/back/remove covering the "did the current photo actually
change" distinction (removing a queued-but-not-current photo bumps
photos_removed but not photos_displayed).

server/app/templates/base.html

@@ -145,7 +145,7 @@
     .icon-btn:hover { background: var(--surface-alt); }
     .icon-btn:active { transform: scale(0.94); }
 
-    h2.card-title {
+    h2.card-title, summary.card-title {
       font-size: 14.5px;
       font-weight: 650;
       margin: 0 0 14px;
@@ -153,6 +153,9 @@
       text-transform: uppercase;
       color: var(--text-muted);
     }
+    summary.card-title { cursor: pointer; margin-bottom: 0; }
+    details.card[open] summary.card-title { margin-bottom: 14px; }
+    details.card .sub { margin-top: 8px; }
 
     .card {
       background: var(--surface);