action.openPopup() is unreliable when called from the background script
during an async message handler — it requires a user gesture context.
tx and sign approvals are triggered programmatically by dApp RPC calls,
not by user clicking the toolbar icon, so openPopup() fails silently.
Use windows.create() directly for tx/sign approvals, matching the
standard extension pattern (used by MetaMask and others). Site-connection
approvals retain openPopup() since they can fall back to the user
clicking the toolbar icon.
Also updates popup window dimensions to 360x600 to match the standard
popup viewport specified in README.
Closes#4
