feat(browse): extend GSTACK_CHROMIUM_PATH support to headless launch

[shohu]

Summary

launchHeaded() already reads GSTACK_CHROMIUM_PATH to select a custom Chromium binary, but launch() (headless mode — the default $B goto path) ignores the env var and always falls back to Playwright's bundled binary.

This PR applies the same pattern to launch():

• If GSTACK_CHROMIUM_PATH is set and the path exists (existsSync guard), it is passed as executablePath to chromium.launch()
• If unset or the binary is missing, falls back to standard Playwright Chromium automatically — no breaking change
• Logs [browse] Using custom Chromium: <path> on startup when active (matches the debug style used elsewhere)

Motivation

This enables drop-in use of stealth Chromium forks (e.g. CloakBrowser) for headless automation without needing to fork gstack or patch vendored files after every upgrade.

Before: GSTACK_CHROMIUM_PATH only worked for headed mode ($B connect). Headless $B goto always used Playwright's bundled binary regardless.

After: Setting GSTACK_CHROMIUM_PATH in the environment applies to both headless and headed mode consistently.

Usage

# ~/.zshrc
export GSTACK_CHROMIUM_PATH="$(python3 -c 'from cloakbrowser import binary_info; print(binary_info()["binary_path"])')"

# Verify: navigator.webdriver should be false with CloakBrowser
$B goto https://example.com
$B js 'navigator.webdriver'   # → false

Test plan

• bun run build passes
• $B goto with GSTACK_CHROMIUM_PATH set → navigator.webdriver: false, window.chrome: object
• $B goto without GSTACK_CHROMIUM_PATH → falls back to standard Playwright Chromium (existsSync guard)
• No changes to headed mode (launchHeaded) behaviour

🤖 Generated with Claude Code

[jbetala7]

One correctness edge with the headless path: this applies GSTACK_CHROMIUM_PATH to launch(), but the extension branch above still unconditionally appends --disable-extensions-except / --load-extension whenever BROWSE_EXTENSIONS_DIR is set. launchHeaded() has an isCustomChromium() guard for this exact case because GBrowser / GStack Browser builds already bake the extension in, and loading it twice can hit the ServiceWorkerState::SetWorkerId DCHECK crash. With this PR, a user running headless/off-screen via BROWSE_EXTENSIONS_DIR plus GSTACK_CHROMIUM_PATH to a baked-extension Chromium can still take the double-load path.

Can this mirror launchHeaded() by skipping the extension flags when isCustomChromium() is true, with a regression around GSTACK_CHROMIUM_KIND=custom-extension-baked or a GBrowser path? That would keep the new headless custom-binary support aligned with the existing headed safety path.