Fix: Make clipboard success reporting reliable

[google-labs-jules]

This commit fixes a bug where the extension would report a clipboard copy failure even when the operation succeeded. The issue was caused by unreliable message passing between the background script and the offscreen document.

The fix implements a more robust communication flow:

1. background.js Changes:

   • A new ensureOffscreenReady() function now pings the offscreen document until it's responsive before any messages are sent.
   • The copyToClipboardViaOffscreen() function was updated to use this readiness check and now includes a retry mechanism, attempting the copy operation twice before failing.

2. offscreen.js Changes:

   • The onMessage listener now correctly returns true to handle asynchronous sendResponse calls, preventing the message port from closing prematurely.
   • A new 'ping' message handler was added to confirm when the document is ready.
   • The clipboard logic is wrapped in an async IIFE to ensure sendResponse is always called.
   • The document.execCommand('copy') fallback now properly checks its boolean return value.

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PR created automatically by Jules for task 16442205392442526423

[google-labs-jules]

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