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/auth.ts

@@ -4,6 +4,7 @@ export type User = {
     id: string;
     username: string;
     created_at: string;
+    is_admin: boolean;
 };
 
 export type Credentials = {