Dashboard customize mode: reorder/hide modules and individual rows, layout saved server-side (GET/PUT /api/prefs/dashboard)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

PLAN.md

@@ -133,7 +133,11 @@ something long in one, switch away and back, refresh — it's all still there.
   from `/sys` (no lm-sensors), per-interface network rates, per-disk read/write/busy
   from `/proc/diskstats`, filesystem usage, UPS via `sudo -n pwrstat -status`. Sampled
   every 2 s into an in-memory ring of 10 min; the UI seeds from
-  `GET /api/metrics/history` then polls `GET /api/metrics`. Still to do: persistent
+  `GET /api/metrics/history` then polls `GET /api/metrics`. **Customizable**: a
+  Customize mode reorders and hides modules (CPU, Memory, Load, Uptime, Temperatures,
+  Fans, Network, Disks, Filesystems, UPS) and hides individual rows (an interface, a
+  mount, a disk, a sensor); the layout is stored server-side in `state.json` via
+  `GET/PUT /api/prefs/dashboard` so every device sees the same dashboard. Still to do: persistent
   history (SQLite ring, e.g. 10 s × 7 days), SMART (`smartctl -j`), longer time ranges.
 - **Multi-host** — same binary as a headless *agent* on `grid` / Mac mini /
   `wyse-5070-1`, `core`'s instance is the UI and fans out over mTLS. The rail grows a