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/local-homeserver.spec.ts

@@ -28,7 +28,7 @@ test.describe('local homeserver regression', () => {
     bob = await ensureUser(uniq('e2e_bob_'));
   });
 
-  test('logs in, opens a room, sends and receives', async ({ page }) => {
+  test('logs in, opens a room, sends and receives @webkit @ios', async ({ page }) => {
     const console_ = collectConsole(page);
     const room = await createRoom(alice, 'Regression Room', { invite: [bob.userId] });
     await joinRoom(bob, room);
@@ -140,7 +140,7 @@ test.describe('local homeserver regression', () => {
     await expect(page.getByText('the original message')).toBeVisible();
   });
 
-  test('thread panel: opens from the chip and yields the member drawer at 1400px (#218)', async ({
+  test('thread panel: opens from the chip and yields the member drawer at 1400px (#218) @webkit', async ({
     browser,
   }) => {
     const ctx = await browser.newContext({ viewport: { width: 1400, height: 850 } });
@@ -169,7 +169,7 @@ test.describe('local homeserver regression', () => {
     await ctx.close();
   });
 
-  test('timeline image opens the gallery lightbox (#219)', async ({ page }) => {
+  test('timeline image opens the gallery lightbox (#219) @webkit', async ({ page }) => {
     const room = await createRoom(alice, 'Lightbox Room');
     const png = Buffer.from(
       'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAAAABJRU5ErkJggg==',