feat: mail phase 3 - sorting (date/from/subject, asc/desc)

- Add sort_by and sort_order query params to list_mails endpoint
- Add deleted_at IS NULL filter to exclude soft-deleted mails
- Frontend: sort state in Mail.tsx, sort header bar in MailList
- Sort dropdown (Datum/Absender/Betreff) + asc/desc toggle
- fetchMails API accepts optional sortBy/sortOrder params
- i18n: 6 new sort keys (de/en)

frontend/src/i18n/locales/de.json

@@ -553,6 +553,12 @@
     "saveDraft": "Entwurf speichern",
     "editDraft": "Entwurf bearbeiten",
     "draftSaved": "Entwurf gespeichert.",
-    "draft": "Entwurf"
+    "draft": "Entwurf",
+    "sortBy": "Sortieren nach",
+    "sortDate": "Datum",
+    "sortFrom": "Absender",
+    "sortSubject": "Betreff",
+    "sortAsc": "Aufsteigend",
+    "sortDesc": "Absteigend"
   }
 }