Fix inverted success/failure logic in evaluate.js

[sirbleek]

Fixes #126

Problem: evaluate_js in css_browserhook.py does not check exceptionDetails in the DevTools Runtime.evaluate response. When JavaScript throws an exception, the code falls through to res = None. In commit_css_transaction, if res is None: return res treats this as success. Meanwhile, successful execution returns "Undefined" (a string), which triggers the retry loop as a failure. Success and failure are completely inverted.

Result: CSS injection silently fails on every attempt. Logs show “Committing css transaction” with no errors, but nothing renders in the Steam UI.

Fix: Change res = "Undefined" to res = None so successful execution correctly signals success to commit_css_transaction.

Tested on: SteamOS 3.7.25, Decky v3.2.3, CSS Loader 2.1.2