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>

backend/routers/settings_router.py

@@ -30,6 +30,7 @@ class SettingsUpdate(BaseModel):
     private_event_visibility: Optional[str] = None
     group_visible_calendar_id: Optional[int] = None
     default_reminder_minutes: Optional[int] = None  # null = off
+    default_event_duration_minutes: Optional[int] = None
 
 
 def _settings_dict(s: models.UserSettings) -> dict:
@@ -52,6 +53,7 @@ def _settings_dict(s: models.UserSettings) -> dict:
         "private_event_visibility": s.private_event_visibility or "busy",
         "group_visible_calendar_id": s.group_visible_calendar_id,
         "default_reminder_minutes": s.default_reminder_minutes,
+        "default_event_duration_minutes": s.default_event_duration_minutes or 60,
     }