feat(admin): phase 8 — unsaved-changes guard on all detail panels

Add beforeNavigate + isDirty tracking to users/[id], users/new,
groups/[id], groups/new, and tags/[id] edit panels. When a user
navigates away with unsaved changes, the navigation is cancelled and
an inline amber warning banner appears with a Discard button that
resumes navigation. Saving successfully clears the dirty flag.

Add i18n key admin_unsaved_warning (de/en/es).
Add spec files for groups/[id] and tags/[id] panels.

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

frontend/src/routes/admin/groups/[id]/page.svelte.spec.ts

@@ -0,0 +1,113 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import Page from './+page.svelte';
+
+vi.mock('$app/forms', () => ({ enhance: () => () => {} }));
+vi.mock('$app/navigation', () => ({ beforeNavigate: vi.fn(), goto: vi.fn() }));
+
+import { beforeNavigate, goto } from '$app/navigation';
+
+const baseGroup = { id: 'g1', name: 'Editoren', permissions: ['WRITE_ALL'] };
+const baseData = { group: baseGroup };
+
+afterEach(cleanup);
+
+// ─── Rendering ────────────────────────────────────────────────────────────────
+
+describe('Admin edit group page – rendering', () => {
+	it('renders the heading with group name', async () => {
+		render(Page, { data: baseData, form: null });
+		await expect.element(page.getByText(/Gruppe: Editoren/i)).toBeInTheDocument();
+	});
+
+	it('pre-fills the name input', async () => {
+		render(Page, { data: baseData, form: null });
+		const input = document.querySelector<HTMLInputElement>('input[name="name"]');
+		expect(input?.value).toBe('Editoren');
+	});
+
+	it('pre-checks permissions that the group already has', async () => {
+		render(Page, { data: baseData, form: null });
+		const checkbox = document.querySelector<HTMLInputElement>(
+			'input[type="checkbox"][name="permissions"][value="WRITE_ALL"]'
+		);
+		expect(checkbox?.checked).toBe(true);
+	});
+
+	it('renders the cancel link pointing to /admin/groups', async () => {
+		render(Page, { data: baseData, form: null });
+		await expect
+			.element(page.getByRole('link', { name: /Abbrechen/i }))
+			.toHaveAttribute('href', '/admin/groups');
+	});
+});
+
+// ─── Unsaved-changes guard ────────────────────────────────────────────────────
+
+describe('Admin edit group page – unsaved-changes guard', () => {
+	beforeEach(() => vi.clearAllMocks());
+
+	it('does not show unsaved warning initially', async () => {
+		render(Page, { data: baseData, form: null });
+		await expect.element(page.getByText(/ungespeicherte Änderungen/i)).not.toBeInTheDocument();
+	});
+
+	it('cancels navigation and shows warning when form is dirty', async () => {
+		render(Page, { data: baseData, form: null });
+		const [callback] = vi.mocked(beforeNavigate).mock.calls[0];
+
+		document
+			.querySelector<HTMLInputElement>('input[name="name"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		const cancel = vi.fn();
+		callback({ cancel, to: { url: new URL('http://localhost/admin/groups/g2') } });
+
+		expect(cancel).toHaveBeenCalled();
+		await expect.element(page.getByText(/ungespeicherte Änderungen/i)).toBeInTheDocument();
+	});
+
+	it('does not cancel navigation when form is clean', async () => {
+		render(Page, { data: baseData, form: null });
+		const [callback] = vi.mocked(beforeNavigate).mock.calls[0];
+
+		const cancel = vi.fn();
+		callback({ cancel, to: { url: new URL('http://localhost/admin/groups/g2') } });
+
+		expect(cancel).not.toHaveBeenCalled();
+	});
+
+	it('discard button calls goto with the target URL', async () => {
+		render(Page, { data: baseData, form: null });
+		const [callback] = vi.mocked(beforeNavigate).mock.calls[0];
+
+		document
+			.querySelector<HTMLInputElement>('input[name="name"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		callback({ cancel: vi.fn(), to: { url: new URL('http://localhost/admin/groups/g2') } });
+
+		await page.getByRole('button', { name: /verwerfen/i }).click();
+
+		expect(vi.mocked(goto)).toHaveBeenCalledWith('http://localhost/admin/groups/g2');
+	});
+
+	it('clears dirty state when form saves successfully', async () => {
+		const { rerender } = render(Page, { data: baseData, form: null });
+		const [callback] = vi.mocked(beforeNavigate).mock.calls[0];
+
+		document
+			.querySelector<HTMLInputElement>('input[name="name"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		callback({ cancel: vi.fn(), to: { url: new URL('http://localhost/admin/groups/g2') } });
+		await expect.element(page.getByText(/ungespeicherte Änderungen/i)).toBeInTheDocument();
+
+		await rerender({ data: baseData, form: { success: true } });
+
+		const cancel = vi.fn();
+		callback({ cancel, to: { url: new URL('http://localhost/admin/groups/g2') } });
+		expect(cancel).not.toHaveBeenCalled();
+	});
+});