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

package.json

@@ -18,7 +18,7 @@
     "typecheck": "tsc --noEmit",
     "test": "node --import tsx --test $(find src -name '*.test.ts')",
     "test:e2e": "playwright test",
-    "test:e2e:install": "playwright install chromium",
+    "test:e2e:install": "playwright install chromium webkit",
     "prepare": "husky",
     "commit": "git-cz",
     "postinstall": "node scripts/patch-folds.mjs",