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/main.py

@@ -210,6 +210,13 @@ def _migrate():
                 logging.info("Migration: added reminders_enabled to %s", tbl)
             except Exception:
                 pass
+        # Synced default duration (minutes) for newly created events.
+        try:
+            conn.execute(text("ALTER TABLE user_settings ADD COLUMN default_event_duration_minutes INTEGER DEFAULT 60"))
+            conn.commit()
+            logging.info("Migration: added default_event_duration_minutes to user_settings")
+        except Exception:
+            pass
 
 _migrate()