Display/GUI watchdog for the "backlight on, no picture" failure

On PoE sites the MIPI-DSI panel can lose its init on a supply dip while the
RK3566 keeps running; the UI may also hang or exit. Three layers:

- hardware watchdog: systemd RuntimeWatchdogSec=20s (system.conf.d)
- GUI: SdWatchdog pings systemd per swapped frame (WATCHDOG_USEC=30s, enabled
  by the binary so older versions run under the same unit) and writes
  run/qt.frame; buttontask.service gets Restart=always, NotifyAccess=main,
  TimeoutAbortSec=10s
- bt-displaywatch (root, buttontask-displaywatch.service): kernel DRM/DSI
  timeouts, eth link down->up (PoE dip hint), stale UI frames, restart storms
  -> panel re-init (stop UI, fb0 blank 4->0, start UI), repeated -> reboot
  (max 3/day, never during OTA); incident log in /var/log/buttontask/

Manual recovery: hold one finger 10 s in the bottom-left corner (panels are
single-touch). Delivered by OTA via bt-update sync_system; the web service
runs `bt-update sync-system` at start for devices updated by an older
bt-update.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

webconfig/system.conf.d/buttontask-watchdog.conf

@@ -0,0 +1,7 @@
+# Hardware watchdog (RK3566 dw_wdt, /dev/watchdog): if the kernel or systemd
+# hangs, the board reboots by itself instead of sitting with a dark screen.
+# Installed to /etc/systemd/system.conf.d/ by bt-update / device-install.sh;
+# applied with `systemctl daemon-reexec`.
+[Manager]
+RuntimeWatchdogSec=20s
+RebootWatchdogSec=2min