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/routes/admin/+layout.svelte

@@ -0,0 +1,75 @@
+<script lang="ts">
+    import { page } from '$app/stores';
+    let { children } = $props();
+
+    const tabs = [
+        { href: '/admin', label: 'Overview' },
+        { href: '/admin/users', label: 'Users' },
+        { href: '/admin/mangas', label: 'Mangas' },
+        { href: '/admin/system', label: 'System' }
+    ];
+</script>
+
+<div class="admin-frame">
+    <aside aria-label="admin">
+        <h2>Admin</h2>
+        <nav>
+            {#each tabs as t (t.href)}
+                <a
+                    href={t.href}
+                    class:active={$page.url.pathname === t.href}
+                    data-testid={`admin-nav-${t.label.toLowerCase()}`}
+                >
+                    {t.label}
+                </a>
+            {/each}
+        </nav>
+    </aside>
+    <section>
+        {@render children()}
+    </section>
+</div>
+
+<style>
+    .admin-frame {
+        display: grid;
+        grid-template-columns: 12rem 1fr;
+        gap: var(--space-4);
+        align-items: start;
+    }
+    @media (max-width: 600px) {
+        .admin-frame {
+            grid-template-columns: 1fr;
+        }
+    }
+    aside {
+        position: sticky;
+        top: calc(var(--app-header-h) + var(--space-4));
+    }
+    aside h2 {
+        margin: 0 0 var(--space-3) 0;
+        font-size: var(--font-base);
+        color: var(--text-muted);
+        text-transform: uppercase;
+        letter-spacing: 0.04em;
+    }
+    nav {
+        display: flex;
+        flex-direction: column;
+        gap: var(--space-1);
+    }
+    nav a {
+        padding: var(--space-2) var(--space-3);
+        border-radius: var(--radius-md);
+        color: var(--text);
+        font-size: var(--font-sm);
+    }
+    nav a:hover {
+        background: var(--surface-elevated);
+        text-decoration: none;
+    }
+    nav a.active {
+        background: var(--surface-elevated);
+        font-weight: var(--weight-semibold);
+    }
+</style>