v0.168.0: customer-configurable backup window (Mentési időablak)

ONE setting (window start W) drives every nightly leg at fixed, never-stored
offsets: DB dump at W, tier-2 at W+60m, off-box at W+105m (wrap-safe). Precedence
settings > controller.yaml db_dump_schedule > 02:30.

- scheduler.UpdateDaily: retime a daily job at runtime (no restart) via a per-job
  buffered resched chan + a select case in runDailyJob.
- new pure package internal/backupwindow (LegTimes/GateWindow/EffectiveWindow).
- quiesce disk-tier window gate: scheduled cycles run only inside [W+2h,W+6h) with a
  safety valve (age>cadence+24h runs regardless); manual TriggerNow never gated.
  Backend.Due now also returns the backup age (from the agent's own /backup/due).
- backup page: Mentési időablak card (time input + derived leg/gate rows); POST
  /backups/window validates -> saves -> UpdateDaily x3 -> flash.

Tests: 5 groups, all red-proofed. Agent/cadence//backup/due untouched.

controller/internal/web/handlers.go

@@ -174,7 +174,7 @@ func (s *Server) dashboardHandler(w http.ResponseWriter, r *http.Request) {
 	// Backup status
 	data["BackupEnabled"] = s.cfg.Backup.Enabled
 	if s.backupMgr != nil {
-		nextDBDump := scheduler.NextDailyRun(s.cfg.Backup.DBDumpSchedule)
+		nextDBDump := scheduler.NextDailyRun(s.effectiveBackupWindow())
 		fullStatus := s.backupMgr.GetFullStatus(nextDBDump)
 		data["DBDumpStatus"] = fullStatus.LastDBDump
 		// F3 (AUDIT-vacation-remote-ops-2026-07-20): the card's "Utolsó mentés" row branches on
@@ -731,7 +731,7 @@ func (s *Server) backupsCommonData(page, title string, r *http.Request) map[stri
 		data["Backup"] = nil
 		return data
 	}
-	nextDBDump := scheduler.NextDailyRun(s.cfg.Backup.DBDumpSchedule)
+	nextDBDump := scheduler.NextDailyRun(s.effectiveBackupWindow())
 	fullStatus := s.backupMgr.GetFullStatus(nextDBDump)
 
 	// Pass flash messages from query params (set by redirect handlers)
@@ -818,6 +818,9 @@ func (s *Server) backupsHandler(w http.ResponseWriter, r *http.Request) {
 	// Whole-guest backup view (agent-sourced, read-only) for the "Rendszermentés" section.
 	data["GuestBackup"] = s.loadGuestBackup(r.Context())
 
+	// Customer-configurable backup window (v0.168.0): effective start + derived leg/gate times.
+	s.backupWindowData(data)
+
 	if fullStatus, ok := data["Backup"].(*backup.FullBackupStatus); ok && fullStatus != nil {
 		// DB-section state — honest messaging for embedded-DB-only boxes (SQLite etc.):
 		// "dumps" (real dumps) | "pending" (discovered, first run tonight) | "embedded".