feat(wireguard): selektiver .conf-Import — Auswahl einzelner Interfaces per Modal

- GET /wireguard/importable listet alle .conf-Dateien in /etc/wireguard/
  mit already_in-Flag (bereits in DB)
- POST /wireguard/import nimmt optionale Names-Liste; ohne Namen → alles
- edgeguard-ctl wg-import [iface…] importiert nur die genannten Interfaces
- UI: Checkbox-Modal mit "Alle neuen auswählen" + Einzelauswahl;
  bereits importierte Interfaces disabled + grüner Tag

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

management-ui/src/i18n/locales/en/common.json

@@ -701,6 +701,17 @@
       "online": "Online",
       "offline": "Offline",
       "traffic": "Traffic"
+    },
+    "import": {
+      "btn": "Import .conf",
+      "modalTitle": "Import WireGuard configs from /etc/wireguard",
+      "hint": "Select which .conf files to import. Already-imported interfaces are greyed out.",
+      "selectAll": "Select all new",
+      "alreadyIn": "already imported",
+      "noFiles": "No .conf files found in /etc/wireguard.",
+      "okBtn": "Import selected",
+      "ok": "Imported {{ifaces}} interface(s), {{peers}} peer(s)",
+      "failed": "Import failed"
     }
   },
   "dashboard": {