feat: add reusable app-file-upload component with drag-and-drop and preview

Replace bare <input type="file"> across all upload locations with a
card-based dropzone component via shared partial. Three visual states:
current server file (thumbnail + Replace/Delete), empty dropzone, and
pending file preview (local FileReader thumbnail + metadata). Delete
actions use data-confirm-message for confirmation dialog.

Centralized as templates/partials/app-file-upload.phtml to prevent
markup drift — documented as deliberate exception to plain-HTML inputs
convention in CLAUDE.md and developer checklist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

i18n/default_de.json

@@ -1430,5 +1430,10 @@
   "Username or password not valid": "Benutzername oder Passwort ungültig",
   "Write-only. Leave empty to keep current value.": "Nur Schreibzugriff. Leer lassen, um den aktuellen Wert beizubehalten.",
   "Your account is deactivated.": "Ihr Konto ist deaktiviert.",
-  "LDAP": "LDAP"
+  "LDAP": "LDAP",
+  "Drop file here or click to select": "Datei hierher ziehen oder klicken",
+  "Remove selected file": "Ausgewaehlte Datei entfernen",
+  "Replace": "Ersetzen",
+  "Delete this image?": "Dieses Bild löschen?",
+  "Current image": "Aktuelles Bild"
 }