ci(e2e): run the tagged specs under WebKit as the Safari/iOS proxy (#221)

Two new Playwright projects — 'webkit' (Desktop Safari) for tests tagged
@webkit and 'iphone' (iPhone 14 descriptor) for @ios — covering boot,
login + send/receive, the thread panel and the gallery lightbox. The CI
e2e job installs webkit next to chromium.

WebKit reports handled fetch failures (well-known probes, a wasm fetch cut
short by our own navigation) as page errors with its own wording, so the
benign allowlist now applies to page errors as well.

Locally: 8/8 green twice in a row against the local Synapse.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

e2e/boot.spec.ts

@@ -5,7 +5,7 @@ import { collectConsole } from './helpers';
 // `vite preview` (see playwright.config.ts webServer). No homeserver needed.
 
 test.describe('boot', () => {
-  test('client boots to the login screen without errors', async ({ page }) => {
+  test('client boots to the login screen without errors @webkit @ios', async ({ page }) => {
     const consoleLog = collectConsole(page);
 
     await page.goto('/');
@@ -24,7 +24,7 @@ test.describe('boot', () => {
     expect(consoleLog.unexpected(), 'unexpected console/page errors during boot').toEqual([]);
   });
 
-  test('service worker script is served and registers', async ({ page }) => {
+  test('service worker script is served and registers @webkit @ios', async ({ page }) => {
     const swResponse = await page.request.get('/sw.js');
     expect(swResponse.status(), 'GET /sw.js').toBe(200);
     expect(swResponse.headers()['content-type'] ?? '').toMatch(/javascript/);