fix: schedule the first image-update check on the cron cadence after a restart (#1491)

In cron mode the service armed the first check for two minutes after boot
regardless of the configured schedule, so a restart triggered an out-of-cadence
check (a weekly cron would run on every boot, then follow cron). Arm the first
check at the next cron fire time instead; interval mode keeps its 2-minute
post-boot delay.

backend/src/services/ImageUpdateService.ts

@@ -220,8 +220,11 @@ export class ImageUpdateService {
         if (this.timer) return;
         this.polling = true;
         this.configureFromSettings();
-        // Preserve the existing 2-minute post-boot delay before the first check.
-        this.armNext(ImageUpdateService.STARTUP_DELAY_MS);
+        // Interval mode keeps the 2-minute post-boot delay before the first check.
+        // Cron mode honors its schedule: arm at the next cron fire time so a restart
+        // never triggers an out-of-cadence check (e.g. a weekly cron must not run on
+        // every boot).
+        this.armNext(this.mode === 'cron' ? this.nextDelayMs() : ImageUpdateService.STARTUP_DELAY_MS);
     }
 
     public stop() {