feat(frontend): /admin dashboard with users/mangas/system views (0.41.0)

Adds the SvelteKit /admin route tree backed by the admin endpoints
landed in PR 1-4. Pages: Overview (alerts + summary cards), Users
(list / promote-demote / delete), Mangas (list with sync state +
expandable per-chapter state), System (live disk/mem/cpu bars,
refreshing every 5s).

Security model: the backend's RequireAdmin extractor is the actual
boundary. /admin/+layout.ts calls getSystemStats() at load and
translates the response — 401 → redirect to /login, 403 → throw
SvelteKit error(403) which renders the framework error page. The
header's "Admin" link is hidden unless `session.user?.is_admin`,
but that's UX only.

Carries `is_admin: boolean` through to the frontend User TS type so
the header check works and so admin tables can show role per row.

Vitest covers lib/api/admin.ts (10 tests: list/delete/PATCH for
users, sync-state filter for mangas, nested chapter route, system
disk-nullable case). Playwright is intentionally deferred until the
routes stabilise — admin UI is operator-only and changes shape often
in v0.

frontend/src/lib/api/admin.ts

@@ -0,0 +1,144 @@
+// Admin-only API client. Every endpoint here is guarded by
+// RequireAdmin on the backend (session cookie only — bearer tokens
+// won't reach these routes). 403s thrown here propagate up to the
+// /admin layout, which renders the framework error page.
+
+import { request, type Page } from './client';
+import type { User } from './auth';
+
+// ---- users -----------------------------------------------------------------
+
+export type AdminUsersPage = {
+    items: User[];
+    page: Page;
+};
+
+export type ListAdminUsersOptions = {
+    search?: string;
+    limit?: number;
+    offset?: number;
+};
+
+export async function listAdminUsers(
+    opts: ListAdminUsersOptions = {}
+): Promise<AdminUsersPage> {
+    const params = new URLSearchParams();
+    if (opts.search) params.set('search', opts.search);
+    if (opts.limit != null) params.set('limit', String(opts.limit));
+    if (opts.offset != null) params.set('offset', String(opts.offset));
+    const qs = params.toString();
+    return request<AdminUsersPage>(`/v1/admin/users${qs ? `?${qs}` : ''}`);
+}
+
+export async function deleteAdminUser(id: string): Promise<void> {
+    await request<void>(`/v1/admin/users/${encodeURIComponent(id)}`, {
+        method: 'DELETE'
+    });
+}
+
+export async function setUserAdmin(id: string, isAdmin: boolean): Promise<User> {
+    return request<User>(`/v1/admin/users/${encodeURIComponent(id)}`, {
+        method: 'PATCH',
+        headers: { 'content-type': 'application/json' },
+        body: JSON.stringify({ is_admin: isAdmin })
+    });
+}
+
+// ---- mangas / chapters with sync state -------------------------------------
+
+export type MangaSyncState = 'in_progress' | 'dropped' | 'synced';
+
+export type AdminMangaRow = {
+    id: string;
+    title: string;
+    status: string;
+    cover_image_path: string | null;
+    created_at: string;
+    updated_at: string;
+    sync_state: MangaSyncState;
+    chapter_count: number;
+    latest_seen_at: string | null;
+};
+
+export type AdminMangasPage = {
+    items: AdminMangaRow[];
+    page: Page;
+};
+
+export type ListAdminMangasOptions = {
+    search?: string;
+    syncState?: MangaSyncState;
+    limit?: number;
+    offset?: number;
+};
+
+export async function listAdminMangas(
+    opts: ListAdminMangasOptions = {}
+): Promise<AdminMangasPage> {
+    const params = new URLSearchParams();
+    if (opts.search) params.set('search', opts.search);
+    if (opts.syncState) params.set('sync_state', opts.syncState);
+    if (opts.limit != null) params.set('limit', String(opts.limit));
+    if (opts.offset != null) params.set('offset', String(opts.offset));
+    const qs = params.toString();
+    return request<AdminMangasPage>(`/v1/admin/mangas${qs ? `?${qs}` : ''}`);
+}
+
+export type ChapterSyncState =
+    | 'downloading'
+    | 'dropped'
+    | 'failed'
+    | 'not_downloaded'
+    | 'synced';
+
+export type AdminChapterRow = {
+    id: string;
+    manga_id: string;
+    number: number;
+    title: string | null;
+    page_count: number;
+    created_at: string;
+    sync_state: ChapterSyncState;
+    latest_seen_at: string | null;
+};
+
+export async function listAdminChapters(mangaId: string): Promise<AdminChapterRow[]> {
+    return request<AdminChapterRow[]>(
+        `/v1/admin/mangas/${encodeURIComponent(mangaId)}/chapters`
+    );
+}
+
+// ---- system ----------------------------------------------------------------
+
+export type DiskStats = {
+    total_bytes: number;
+    used_bytes: number;
+    free_bytes: number;
+    percent_used: number;
+};
+
+export type MemoryStats = {
+    total_bytes: number;
+    used_bytes: number;
+    percent_used: number;
+};
+
+export type CpuStats = {
+    percent_used: number;
+};
+
+export type Alert = {
+    level: 'warning';
+    message: string;
+};
+
+export type SystemStats = {
+    disk: DiskStats | null;
+    memory: MemoryStats;
+    cpu: CpuStats;
+    alerts: Alert[];
+};
+
+export async function getSystemStats(): Promise<SystemStats> {
+    return request<SystemStats>('/v1/admin/system');
+}