feat: mail phase 1 - HTML iframe display, bulk read/unread, IMAP flag sync

- Fix FlagUpdatePayload field names to match backend schema (is_seen/is_flagged)
- Replace dangerouslySetInnerHTML with sandboxed iframe (srcDoc, sandbox="")
- Add bulk mark read/unread with checkbox selection in MailList
- Add floating bulk action bar in Mail.tsx
- Add imap_sync_mail_flags() to push Seen/Flagged flags to IMAP server
- Call IMAP flag sync in PATCH /flags endpoint
- Add i18n keys for bulk actions (de/en)

frontend/src/api/mail.ts

@@ -246,8 +246,11 @@ export interface ForwardPayload {
 }
 
 export interface FlagUpdatePayload {
-  seen?: boolean;
-  flagged?: boolean;
+  is_seen?: boolean;
+  is_flagged?: boolean;
+  is_draft?: boolean;
+  is_answered?: boolean;
+  is_forwarded?: boolean;
 }
 
 export interface LinkMailPayload {