feat: safe HTML descriptions, custom reminder picker, synced default event duration

- Render event descriptions as sanitized HTML (links/line breaks) instead of
  raw escaped text; no script execution
- Reminder picker: presets + custom number+unit (minutes/hours/days/weeks)
- Grey out + hint the reminder editor when a calendar's notifications are off
  (reminders are kept, just not fired)
- New synced setting default_event_duration_minutes (default 60) for new events

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

frontend/js/i18n.js

@@ -87,6 +87,7 @@ const translations = {
     settings_week_start: 'Erster Wochentag',
     week_start_monday: 'Montag', week_start_sunday: 'Sonntag',
     settings_dim_past: 'Vergangene Termine ausgrauen',
+    settings_default_duration: 'Standard-Termindauer',
     settings_privacy: 'Privatsphäre',
     settings_private_visibility: 'Private Termine für Gruppenmitglieder',
     settings_private_visibility_desc: 'Wie private Termine für andere Gruppenmitglieder erscheinen',
@@ -104,6 +105,13 @@ const translations = {
     reminder_days: '{n} Tage vorher',
     reminder_week_one: '1 Woche vorher',
     reminder_weeks: '{n} Wochen vorher',
+    reminder_custom: 'Benutzerdefiniert…',
+    reminder_unit_minutes: 'Minuten',
+    reminder_unit_hours: 'Stunden',
+    reminder_unit_days: 'Tage',
+    reminder_unit_weeks: 'Wochen',
+    reminder_before: 'vorher',
+    reminders_calendar_off: 'Für diesen Kalender sind Benachrichtigungen deaktiviert – Erinnerungen werden nicht ausgeführt.',
     calendar_reminders_on: 'Benachrichtigungen aktiviert',
     calendar_reminders_off: 'Benachrichtigungen deaktiviert',
     share: 'Teilen',
@@ -372,6 +380,7 @@ const translations = {
     settings_week_start: 'First day of week',
     week_start_monday: 'Monday', week_start_sunday: 'Sunday',
     settings_dim_past: 'Dim past events',
+    settings_default_duration: 'Default event duration',
     settings_privacy: 'Privacy',
     settings_private_visibility: 'Private events for group members',
     settings_private_visibility_desc: 'How your private events appear to other group members',
@@ -389,6 +398,13 @@ const translations = {
     reminder_days: '{n} days before',
     reminder_week_one: '1 week before',
     reminder_weeks: '{n} weeks before',
+    reminder_custom: 'Custom…',
+    reminder_unit_minutes: 'minutes',
+    reminder_unit_hours: 'hours',
+    reminder_unit_days: 'days',
+    reminder_unit_weeks: 'weeks',
+    reminder_before: 'before',
+    reminders_calendar_off: 'Reminders are disabled for this calendar – they will not fire.',
     calendar_reminders_on: 'Reminders enabled',
     calendar_reminders_off: 'Reminders disabled',
     share: 'Share',