feat: add user theme preference

- Add theme field to User model (common)
- Create migration for theme field
- Add theme selection to Account Settings (html-router)
- Implement server-side theme rendering in base template
- Update JS for system/preference theme handling
- Remove header theme toggle for authenticated users

common/migrations/20260116_000000_add_user_theme_preference.surql

@@ -0,0 +1 @@
+DEFINE FIELD IF NOT EXISTS theme ON user TYPE string DEFAULT "system";