feat: mail list font size, remove Termin toolbar, rename Flagge to Markierung with symbol picker

- Mail list: title and subject font text-xs -> text-sm
- Remove Termin (create-event) toolbar button
- Rename Flagge -> Markierung with select dropdown (star, flag, bookmark, important, question)
- Backend: add flag_type column to Mail model + migration 0006
- Backend: flag_type in MailFlagsUpdate schema, update_flags route, mail_to_response
- Frontend: flag_type in Mail interface, FlagUpdatePayload, MailList symbol display

app/plugins/builtins/mail/services.py

@@ -1562,6 +1562,7 @@ def mail_to_response(
         "date": mail.received_at.isoformat() if mail.received_at else (mail.sent_at.isoformat() if mail.sent_at else None),
         "is_seen": mail.is_seen,
         "is_flagged": mail.is_flagged,
+        "flag_type": mail.flag_type,
         "is_draft": mail.is_draft,
         "is_answered": mail.is_answered,
         "is_forwarded": mail.is_forwarded,