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/models.py

@@ -106,6 +106,8 @@ class UserSettings(Base):
     # Default reminder in minutes-before-start applied to all events client-side
     # (0 = at start time). NULL = no default reminder.
     default_reminder_minutes = Column(Integer, nullable=True)
+    # Default duration (in minutes) applied to a newly created event's end time.
+    default_event_duration_minutes = Column(Integer, default=60)
 
     user = relationship("User", back_populates="settings")