Isolate extension WebView profiles and follow host proxy settings

internal/webview/README.md

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+# Extension browser host configuration
+
+The downloader wraps each provider with the extension's stable identity before
+constructing the runtime. `OpenOptions.ProfileID`, `DataPath`, and `ProxyURL` are
+host-only fields. `parseOpenOptions` never reads them from extension JavaScript.
+The extension API is unchanged.
+
+Profiles are persistent and shared across invocations/pages of the same extension.
+Different identities use different native stores. Go providers use a SHA-256-based
+path under `<StorageDir>/webview`; Apple RPC hosts use a stable UUID data store;
+Android uses a named WebView profile. Cookie operations select that same store,
+including `clearCookies`. Closing an execution closes its pages, not its profile.
+Existing shared browser data is not copied into every extension's new profile.
+
+The downloader lazily owns one loopback forward proxy with HTTP and SOCKS5
+listeners. Windows/Linux/Android use HTTP; Apple uses SOCKS5 because its HTTP
+CONNECT configuration may bypass plain HTTP. The forwarder reads the current
+Gopeed proxy config for each new request/tunnel and supports direct, system,
+HTTP(S) upstream proxies with Basic authentication, and authenticated SOCKS5.
+Credentials remain in Go and are never sent over the WebView RPC.
+
+Changes affect new requests/connections. Existing CONNECT/SOCKS tunnels keep their
+route until closed; opening another page in the same profile does not reset it.
+SOCKS carries host/port rather than a full URL: system proxy/PAC selection for
+Apple is host-based, with port 80 treated as HTTP and other ports as HTTPS.
+Path-dependent PAC rules cannot be reproduced from a SOCKS connection.
+
+Apple requires macOS 14 / iOS 17. Android checks MULTI_PROFILE and PROXY_OVERRIDE.
+Unsupported hosts fail explicitly instead of silently sharing browser storage or
+ignoring the proxy.
+
+## Validation
+
+- `go test -race ./internal/webview/proxy ./pkg/download/engine/webview`
+- `go test ./pkg/download -run 'TestWebViewProfileBound|TestDownloader_ExtensionRuntimeWebView'`
+- In webview_go: `go run ./examples/profilecheck` (Linux: `xvfb-run -a ...`).
+- In ui/flutter: `flutter run -d macos -t tool/webview_profile_check.dart`.
+
+The native smoke tests cover routing and persistent profile isolation. The Flutter
+smoke test also checks concurrent pages, HttpOnly cookies, LocalStorage, IndexedDB,
+profile-scoped clearing, and reopening. The self-signed TLS fixture is trusted only
+by that test's callback; production certificate validation is unchanged.