feat(admin/system): add system maintenance page under /admin/system

Moves the system maintenance panel out of the old tab-based admin page
and into a dedicated route. Renders maintenance cards with spinner state
and success message on completion.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/system/page.svelte.spec.ts

@@ -0,0 +1,34 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import Page from './+page.svelte';
+
+afterEach(cleanup);
+
+describe('Admin system page', () => {
+	it('renders the backfill versions heading', async () => {
+		render(Page, {});
+		await expect.element(page.getByText(/Verlaufsdaten auffüllen/i)).toBeInTheDocument();
+	});
+
+	it('renders the backfill versions button', async () => {
+		render(Page, {});
+		await expect
+			.element(page.getByRole('button', { name: /jetzt auffüllen/i }))
+			.toBeInTheDocument();
+	});
+
+	it('renders the backfill file hashes heading', async () => {
+		render(Page, {});
+		await expect
+			.element(page.getByRole('heading', { name: /Datei-Hashes berechnen/i }))
+			.toBeInTheDocument();
+	});
+
+	it('renders the file hashes button', async () => {
+		render(Page, {});
+		await expect
+			.element(page.getByRole('button', { name: /Datei-Hashes berechnen/i }))
+			.toBeInTheDocument();
+	});
+});