fix: show/enter slot duration in hours instead of minutes in admin schedule

Storage stays in minutes (slot_minutes); the admin form now takes
hours (0.25 step), matching the service-duration input.

src/pages/admin/AdminSchedule.tsx

@@ -153,14 +153,15 @@ function AdminSchedule() {
             />
           </label>
           <label htmlFor="schedule-slot-minutes">
-            Durée des créneaux (minutes)
+            Durée des créneaux (heures)
             <input
               id="schedule-slot-minutes"
               type="number"
-              min="5"
+              step="0.25"
+              min="0.25"
               required
-              value={template.slot_minutes}
-              onChange={(e) => setTemplate({ ...template, slot_minutes: Number(e.target.value) })}
+              value={template.slot_minutes / 60}
+              onChange={(e) => setTemplate({ ...template, slot_minutes: Math.round(Number(e.target.value) * 60) })}
             />
           </label>
           <button type="submit" className={styles.button} disabled={saving}>Enregistrer l&apos;horaire</button>