feat(settings): admin-gated User Management tab via Navidrome REST API

Adds a dedicated Settings tab for managing Navidrome users (list, create,
edit, delete). Only visible to admins — gated by `/auth/login` probe on
mount. Uses Navidrome's native /api/user endpoints instead of Subsonic
(getUsers.view etc. return only the caller on Navidrome). All HTTP calls
go through new Rust commands to bypass WebView CORS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/api/navidromeAdmin.ts

@@ -0,0 +1,65 @@
+import { invoke } from '@tauri-apps/api/core';
+
+export interface NdUser {
+  id: string;
+  userName: string;
+  name: string;
+  email: string;
+  isAdmin: boolean;
+  lastLoginAt?: string | null;
+  createdAt?: string;
+  updatedAt?: string;
+}
+
+export interface NdLoginResult {
+  token: string;
+  userId: string;
+  isAdmin: boolean;
+}
+
+export async function ndLogin(
+  serverUrl: string,
+  username: string,
+  password: string,
+): Promise<NdLoginResult> {
+  return invoke<NdLoginResult>('navidrome_login', { serverUrl, username, password });
+}
+
+export async function ndListUsers(serverUrl: string, token: string): Promise<NdUser[]> {
+  const raw = await invoke<unknown>('nd_list_users', { serverUrl, token });
+  if (!Array.isArray(raw)) return [];
+  return raw.map(u => {
+    const o = u as Record<string, unknown>;
+    return {
+      id: String(o.id ?? ''),
+      userName: String(o.userName ?? ''),
+      name: String(o.name ?? ''),
+      email: String(o.email ?? ''),
+      isAdmin: !!o.isAdmin,
+      lastLoginAt: (o.lastLoginAt as string | null | undefined) ?? null,
+      createdAt: o.createdAt as string | undefined,
+      updatedAt: o.updatedAt as string | undefined,
+    };
+  });
+}
+
+export async function ndCreateUser(
+  serverUrl: string,
+  token: string,
+  data: { userName: string; name: string; email: string; password: string; isAdmin: boolean },
+): Promise<void> {
+  await invoke('nd_create_user', { serverUrl, token, ...data });
+}
+
+export async function ndUpdateUser(
+  serverUrl: string,
+  token: string,
+  id: string,
+  data: { userName: string; name: string; email: string; password: string; isAdmin: boolean },
+): Promise<void> {
+  await invoke('nd_update_user', { serverUrl, token, id, ...data });
+}
+
+export async function ndDeleteUser(serverUrl: string, token: string, id: string): Promise<void> {
+  await invoke('nd_delete_user', { serverUrl, token, id });
+}