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/i18n/locales/de.json

@@ -535,6 +535,11 @@
     "tabRules": "Regeln",
     "tabLabels": "Labels",
     "tabVacation": "Abwesenheit",
-    "tabPgp": "PGP-Verschlüsselung"
+    "tabPgp": "PGP-Verschlüsselung",
+    "markRead": "Als gelesen markieren",
+    "markUnread": "Als ungelesen markieren",
+    "selectAll": "Alle auswählen",
+    "bulkActions": "Sammelaktionen",
+    "selectedCount": "{{count}} ausgewählt"
   }
 }