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/users/[id]/page.svelte.spec.ts

@@ -1,9 +1,12 @@
-import { afterEach, describe, expect, it, vi } from 'vitest';
+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 groups = [
 	{ id: 'g1', name: 'Editoren', permissions: ['WRITE_ALL'] },
@@ -141,3 +144,72 @@ describe('Admin edit user page – feedback', () => {
 		await expect.element(page.getByText(/Änderungen gespeichert/i)).not.toBeInTheDocument();
 	});
 });
+
+// ─── Unsaved-changes guard ────────────────────────────────────────────────────
+
+describe('Admin edit user 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="firstName"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		const cancel = vi.fn();
+		callback({ cancel, to: { url: new URL('http://localhost/admin/users/u2') } });
+
+		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/users/u2') } });
+
+		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="firstName"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		callback({ cancel: vi.fn(), to: { url: new URL('http://localhost/admin/users/u2') } });
+
+		await page.getByRole('button', { name: /verwerfen/i }).click();
+
+		expect(vi.mocked(goto)).toHaveBeenCalledWith('http://localhost/admin/users/u2');
+	});
+
+	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="firstName"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		callback({ cancel: vi.fn(), to: { url: new URL('http://localhost/admin/users/u2') } });
+		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/users/u2') } });
+		expect(cancel).not.toHaveBeenCalled();
+	});
+});