feat: custom reminder picker, muted-calendar hint, synced default duration

- Reminder editor: presets + custom number+unit (minutes/hours/days/weeks)
- Grey out + footer hint when the selected calendar's reminders are muted;
  reminders are kept, scheduler already skips them
- New synced setting defaultEventDurationMinutes for new events

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Calendarr iOS/Models/Localization.swift

@@ -127,6 +127,7 @@ private let strings: [String: [String: String]] = [
         "settings.monday": "Montag",
         "settings.sunday": "Sonntag",
         "settings.dimpast": "Vergangene Termine ausgrauen",
+        "settings.default_duration": "Standard-Termindauer",
         "settings.nav.profile": "Profil",
         "settings.privacy": "Privatsphäre",
         "settings.private_visibility": "Private Termine für Gruppen",
@@ -455,6 +456,7 @@ private let strings: [String: [String: String]] = [
         "settings.monday": "Monday",
         "settings.sunday": "Sunday",
         "settings.dimpast": "Dim past events",
+        "settings.default_duration": "Default event duration",
         "settings.nav.profile": "Profile",
         "settings.privacy": "Privacy",
         "settings.private_visibility": "Private events for groups",