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

frontend/src/api/mail.ts

@@ -65,6 +65,7 @@ export interface Mail {
   date: string;
   is_seen: boolean;
   is_flagged: boolean;
+  flag_type?: string | null;
   is_draft: boolean;
   is_answered: boolean;
   has_attachments: boolean;
@@ -263,6 +264,7 @@ export interface ForwardPayload {
 export interface FlagUpdatePayload {
   is_seen?: boolean;
   is_flagged?: boolean;
+  flag_type?: string | null;
   is_draft?: boolean;
   is_answered?: boolean;
   is_forwarded?: boolean;