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/schemas.py

@@ -145,6 +145,7 @@ class MailForwardRequest(BaseModel):
 class MailFlagsUpdate(BaseModel):
     is_seen: bool | None = None
     is_flagged: bool | None = None
+    flag_type: str | None = None
     is_draft: bool | None = None
     is_answered: bool | None = None
     is_forwarded: bool | None = None
@@ -166,6 +167,7 @@ class MailResponse(BaseModel):
     body_html_sanitized: str
     is_seen: bool
     is_flagged: bool
+    flag_type: str | None = None
     is_draft: bool
     is_answered: bool
     is_forwarded: bool