v0.4.5: Add dedicated Backup page (Biztonsági mentés)

New /backups page with full backup system visibility:
- Status overview cards (local/remote backup, DB count, repo size)
- Schedule section with next-run times and retention policy
- Database table with type, size, validation (table count), status
- Snapshot history table with per-snapshot stats
- Repository info card with paths, integrity status, remote placeholder
- "Mentés most" button with auto-refresh polling
- Empty state when backup not configured

Backend: SnapshotRecord history (ring buffer), DumpValidation,
ListDumpFiles, ListSnapshots, GetFullStatus, restic check tracking.
Server accepts scheduler for next-run time calculation.

Sidebar nav updated with 3rd item, dashboard backup card title clickable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/scheduler/scheduler.go

@@ -225,6 +225,11 @@ func parseDailyTime(timeStr string) (int, int, error) {
 	return hour, min, nil
 }
 
+// NextDailyRun is the exported version of nextDailyRun for external callers.
+func NextDailyRun(timeStr string) time.Time {
+	return nextDailyRun(timeStr)
+}
+
 // nextDailyRun calculates the next occurrence of the daily schedule in Europe/Budapest timezone.
 func nextDailyRun(timeStr string) time.Time {
 	hour, min, err := parseDailyTime(timeStr)