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/templates/backups.html

@@ -109,6 +109,26 @@
 </div>
 {{end}}
 
+<!-- Section: customer-configurable backup window (v0.168.0) — one setting drives every nightly leg -->
+<div class="backup-section-card">
+    <h3>Mentési időablak</h3>
+    <p class="form-hint" style="margin-bottom:1rem">A mentések egymás után futnak: adatbázis-mentés, helyi másolat, távoli mentés, majd a teljes rendszermentés.</p>
+    <form method="POST" action="/backups/window" class="schedule-actions" style="display:flex;align-items:flex-end;gap:.75rem;flex-wrap:wrap">
+        {{.CSRFField}}
+        <div class="form-group" style="margin:0">
+            <label for="window_start">Mentési időablak kezdete</label>
+            <input type="time" id="window_start" name="window_start" value="{{.BackupWindow}}" class="form-control" style="max-width:9rem" required>
+        </div>
+        <button type="submit" class="btn btn-sm btn-primary">Mentés</button>
+    </form>
+    <ul class="form-hint" style="margin-top:1rem;margin-bottom:0;list-style:none;padding:0;line-height:1.9">
+        <li>Adatbázis-mentés: <strong>{{.BackupLegDB}}</strong></li>
+        <li>Helyi másolat: <strong>{{.BackupLegTier2}}</strong></li>
+        <li>Távoli mentés: <strong>{{.BackupLegOffbox}}</strong></li>
+        <li>Teljes rendszermentés: kb. <strong>{{.BackupGateFrom}}–{{.BackupGateTo}}</strong> között</li>
+    </ul>
+</div>
+
 <!-- Section 1: Status overview cards -->
 <div class="stats-grid backup-page-cards">
     {{if .Backup.LastDBDump}}