chore: add cross-platform webdriver tests

e2e/README.md

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+# Donut Browser native E2E tests
+
+These tests exercise the actual Tauri application through the private
+`../tauri-cross-platform-webdriver/` driver. They do not replace Rust or React unit tests; they
+cover the process boundaries those tests cannot: WKWebView/WebView2/WebKitGTK UI, Tauri invokes,
+REST and MCP servers, two-device sync, S3 payload encryption, Wayfern, CDP, and child-process
+cleanup.
+
+## Local setup
+
+Place both repositories beside each other:
+
+```text
+Code/
+├── donutbrowser/
+└── tauri-cross-platform-webdriver/
+```
+
+Install Donut dependencies with `pnpm install`. The browser suite also needs
+`WAYFERN_TEST_TOKEN`. The runner reads it from the environment, Donut's `.env`, or
+`../wayfern-test/.env` without printing it. If `../wayfern-test/test_extracted_app` contains a
+Wayfern build, the runner links/copies it into the test data root; otherwise the browser suite
+downloads the current published build into that root.
+
+Run one suite:
+
+```sh
+pnpm e2e:smoke
+pnpm e2e:ui
+pnpm e2e:entities
+pnpm e2e:integrations
+pnpm e2e:sync
+pnpm e2e:browser
+```
+
+Run everything with `pnpm e2e`. A normal run builds the Next frontend, `donut-proxy`, the
+`e2e`-feature app, and `tauri-wd`. Add `--no-build` to `node e2e/run.mjs --suite=<name>` only when
+all four outputs are current. `DONUT_E2E_KEEP_ARTIFACTS=1` retains successful runs; failed runs are
+always retained and their location is printed.
+
+## Isolation contract
+
+Each app session receives a unique root under the operating-system test temp directory. The
+runner redirects:
+
+- Donut data, cache, and logs with `DONUTBROWSER_DATA_ROOT`;
+- `HOME`, `USERPROFILE`, `CFFIXED_USER_HOME`, XDG paths, `APPDATA`, and `LOCALAPPDATA`;
+- `TMPDIR`, `TMP`, and `TEMP`;
+- the Tauri WebView store (incognito for WKWebView, whose persistent data-directory API is not
+  honored);
+- all REST, MCP, WebDriver, fixture, MinIO, and sync-server ports;
+- each sync test to a new MinIO bucket and random token.
+
+The E2E feature suppresses automatic updater/download traffic, but explicit browser tests still
+exercise published Wayfern downloads when no local fixture exists. Entitlement fallback from
+`WAYFERN_TEST_TOKEN` exists only in the feature-gated test binary. Production builds never include
+the WebDriver plugin or this fallback.
+
+## CI
+
+`.github/workflows/app-e2e.yml` runs smoke tests on macOS, Linux/Xvfb, and Windows for pull
+requests. Pushes to `main`, weekly schedules, and manual runs execute the full macOS suite,
+including MinIO-backed sync and real Wayfern automation.
+
+Because the WebDriver repository is private, CI needs a `TAURI_WEBDRIVER_TOKEN` secret with
+read-only access. `TAURI_WEBDRIVER_REPOSITORY` may override its default
+`zhom/tauri-cross-platform-webdriver` repository name. The full job also requires the
+`WAYFERN_TEST_TOKEN` secret.