feat(plex): P0 admin Config UI — connection test + library picker

- ConfigPanel 'plex' group auto-renders; adds a Test-connection button + a
  library-picker (checked sections write plex_libraries, applied on Save)
- api.testPlex + PlexTestResult/PlexSection types
- i18n config namespace: plex group + 7 fields + test strings (en/hu/de);
  also fills the previously-missing 'downloads' group label

frontend/src/lib/api.ts

@@ -609,6 +609,20 @@ export interface SystemConfigData {
   secrets_manageable: boolean;
 }
 
+// Admin: result of testing the Plex server connection (also drives the library-picker).
+export interface PlexSection {
+  key: string;
+  title: string;
+  type: string; // "movie" | "show"
+  count?: number;
+}
+export interface PlexTestResult {
+  ok: boolean;
+  server_name: string;
+  version?: string;
+  sections: PlexSection[];
+}
+
 // Admin Users & roles page.
 export interface AdminUserRow {
   id: number;
@@ -921,6 +935,7 @@ export const api = {
     req(`/api/admin/config/${key}`, { method: "DELETE" }),
   testEmail: (): Promise<{ sent: boolean; to: string }> =>
     req("/api/admin/config/test-email", { method: "POST" }),
+  testPlex: (): Promise<PlexTestResult> => req("/api/plex/test", { method: "POST" }),
   // --- admin: users & roles ---
   adminUsers: (): Promise<AdminUserRow[]> => req("/api/admin/users"),
   setUserRole: (id: number, role: "user" | "admin"): Promise<AdminUserRow> =>