fix: fall back when Clipboard API write fails

[Hexagon0000]

Summary

• Clipboard copy currently swallows navigator.clipboard.writeText failures and still shows the success checkmark, so users think feedback was copied when the clipboard is empty.
• This is easy to hit in embedded browsers / unfocused documents (NotAllowedError: Document is not focused) and other non-secure or permission-restricted contexts.
• Add copyTextToClipboard with an execCommand('copy') fallback, and only set the copied UI state when a write actually succeeds (while still always invoking onCopy).

Test plan

• Unit tests for clipboard helper: API success, API fail → fallback success, both fail
• Manual: add an annotation → Copy feedback → paste elsewhere matches markdown
• Manual (if possible): block Clipboard API and confirm fallback still copies
• Manual: when both paths fail, success checkmark does not appear; onCopy still fires

Made with Cursor

[vercel]

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