Previously the welcome page arrow hid immediately when the extension's
content script loaded — but extension loaded ≠ sidebar open. Now the
signal flow is: sidepanel connects → tells background.js → relays to
content script → dispatches gstack-extension-ready → arrow hides.
Adds welcome-page.test.ts: 14 tests verifying arrow, branding, feature
cards, dark theme, and auto-hide behavior via real HTTP server.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>