fix(pi-extension): guard theme access with try-catch for pi-web compatibility (#336)

pi-coding-agent 0.80.x uses a Proxy for the theme object that throws
'Theme not initialized' when accessed before initTheme(). In pi-web,
the session_start event fires before the theme is ready, causing
the optional chaining guard to fail (Proxy is truthy, ?. can't
catch Proxy get-trap throws).

Wrap theme access in try-catch so the extension degrades gracefully.

Co-authored-by: huantian666 <huantian666@gmail.com>
Co-authored-by: Emeriko <dietrich.gebert@gmail.com>