refactor(web): sweep 8 test files to use pageUrlModule() (#375)

Centralizes the inline { page: { get url() { return state.pageUrl } } }
mock shape via test-utils/mocks/appState.ts. The vi.hoisted state
declaration remains per-file (vitest mock-hoisting requires module-
level declaration). Adds a $test-utils alias to svelte.config.js so
the helper can be imported without ../../../ chains.

SKIPPED (mock exposes more than url):
- Shell.test.ts, MobileNavDrawer.test.ts, admin.test.ts: static
  page: { url: ... } shape, no hoisted state to centralize.
- album.test.ts, artist.test.ts: page exposes both params and url.
- playlist.test.ts, reset-password.test.ts: page exposes params only.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/AdminTabs.test.ts

@@ -1,13 +1,12 @@
 import { describe, expect, test, vi } from 'vitest';
 import { render, screen } from '@testing-library/svelte';
+import { pageUrlModule } from '$test-utils/mocks/appState';
 
 const state = vi.hoisted(() => ({
   pageUrl: new URL('http://localhost/admin')
 }));
 
-vi.mock('$app/state', () => ({
-  page: { get url() { return state.pageUrl; } }
-}));
+vi.mock('$app/state', () => pageUrlModule(state));
 
 import AdminTabs from './AdminTabs.svelte';
 

web/src/lib/components/DiscoverTabs.test.ts

@@ -1,13 +1,12 @@
 import { describe, expect, test, vi } from 'vitest';
 import { render, screen } from '@testing-library/svelte';
+import { pageUrlModule } from '$test-utils/mocks/appState';
 
 const state = vi.hoisted(() => ({
   pageUrl: new URL('http://localhost/discover')
 }));
 
-vi.mock('$app/state', () => ({
-  page: { get url() { return state.pageUrl; } }
-}));
+vi.mock('$app/state', () => pageUrlModule(state));
 
 import DiscoverTabs from './DiscoverTabs.svelte';
 

web/src/lib/components/SearchInput.test.ts

@@ -1,13 +1,12 @@
 import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
 import { render, screen, fireEvent } from '@testing-library/svelte';
+import { pageUrlModule } from '$test-utils/mocks/appState';
 
 const state = vi.hoisted(() => ({
   pageUrl: new URL('http://localhost/')
 }));
 
-vi.mock('$app/state', () => ({
-  page: { get url() { return state.pageUrl; } }
-}));
+vi.mock('$app/state', () => pageUrlModule(state));
 
 vi.mock('$app/navigation', () => ({
   goto: vi.fn()