feat(frontend): invite-based registration UI

- Add /register route with invite code prefill, password show/hide
- Add /login?registered=1 success banner
- Add /admin/invites page: list, create, revoke, copy link
- Add Einladungen nav section to admin sidebar (ADMIN_USER perm)
- Add invite error codes to errors.ts
- Add 48 i18n keys across de/en/es
- Update hooks.server.ts to allow public access to invite/register API

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

frontend/src/routes/admin/layout.server.spec.ts

@@ -56,14 +56,20 @@ describe('admin layout load — permission check', () => {
 			[{ id: 't1' }, { id: 't2' }, { id: 't3' }]
 		);
 
+		const mockFetch = vi.fn().mockResolvedValue({
+			ok: true,
+			json: async () => [{ id: 'i1' }, { id: 'i2' }]
+		});
+
 		const result = await load({
-			fetch: vi.fn() as unknown as typeof fetch,
+			fetch: mockFetch as unknown as typeof fetch,
 			locals: { user: adminUser }
 		});
 
 		expect(result.userCount).toBe(2);
 		expect(result.groupCount).toBe(1);
 		expect(result.tagCount).toBe(3);
+		expect(result.inviteCount).toBe(2);
 		expect(result.canManageUsers).toBe(true);
 		expect(result.canManageTags).toBe(true);
 		expect(result.canManagePermissions).toBe(true);