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>

qml/Main.qml

@@ -180,6 +180,18 @@ Window {
         onDoubleClicked: passwordDialog.open()
     }
 
+    // Display recovery corner: holding it 10 s re-inits the panel (detected in
+    // C++ RecoveryGesture). This area only shields buttons underneath; a
+    // button reaching into the corner does not react to touches there.
+    MouseArea {
+        id: recoveryHotspot
+        anchors.left: parent.left
+        anchors.bottom: parent.bottom
+        width: 72
+        height: 72
+        z: 20
+    }
+
     Connections {
         target: settingsContainer
         onChanged: root.hideSensorsIfDisabled()