feat: mail phase 2 - delete, move, drafts

- Add deleted_at field to Mail/MailAccount/MailFolder models
- Add DELETE /{mail_id} endpoint (soft-delete + IMAP EXPUNGE)
- Add POST /{mail_id}/move endpoint (IMAP UID MOVE + folder_id update)
- Add POST /drafts and PUT /drafts/{id} endpoints (save/edit drafts)
- Add imap_delete_mail() and imap_move_mail() service functions
- Add save_draft() and update_draft() service functions
- Frontend: deleteMail, moveMail, saveDraft, updateDraft API functions
- ComposeModal: draft mode with Save Draft button
- MailDetail: delete/move buttons, edit-draft for drafts
- Mail.tsx: bulk delete/move, move dropdown, draft handlers
- i18n: 13 new keys (de/en)

frontend/src/i18n/locales/en.json

@@ -540,6 +540,19 @@
     "markUnread": "Mark as Unread",
     "selectAll": "Select All",
     "bulkActions": "Bulk Actions",
-    "selectedCount": "{{count}} selected"
+    "selectedCount": "{{count}} selected",
+    "delete": "Delete",
+    "deleteConfirm": "Are you sure you want to delete this email?",
+    "deleteMail": "Delete Email",
+    "bulkDelete": "Delete All",
+    "bulkDeleteConfirm": "Are you sure you want to delete the selected emails?",
+    "move": "Move",
+    "moveTo": "Move to",
+    "moveMail": "Move Email",
+    "bulkMove": "Move All",
+    "saveDraft": "Save Draft",
+    "editDraft": "Edit Draft",
+    "draftSaved": "Draft saved.",
+    "draft": "Draft"
   }
 }