fix: parseOsc correctly reports ClipboardEvent selection on base64 failure

[masterkeysrd]

Previously, the OSC 52 parser used strings.Split, which could corrupt the payload if the base64 content contained semicolons. It also required a non-empty selection code, so empty selections silently dropped the event, and on invalid base64 it returned the raw, un-decoded string as the content instead of an empty string — causing the wrong selection and content to be reported in ClipboardEvent.

Before passing an empty clipboard event

After my changes, now the base64 is correctly populated in the Clipboard Event

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).