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/routes.py

@@ -1290,6 +1290,8 @@ async def update_flags(
         mail.is_seen = data.is_seen
     if data.is_flagged is not None:
         mail.is_flagged = data.is_flagged
+    if data.flag_type is not None:
+        mail.flag_type = data.flag_type if data.flag_type else None
     if data.is_draft is not None:
         mail.is_draft = data.is_draft
     if data.is_answered is not None: