feat: show export file path with copy button in toast notification

[j4rviscmd]

Summary

• Display the full export destination path in the toast body when exporting a note as Markdown
• Add a copy-to-clipboard button (left of the path) that briefly shows a checkmark icon on success
• Path text wraps naturally for long paths instead of truncating

Test plan

• Export a note as Markdown and verify the toast shows the full file path
• Click the copy button and verify the icon changes to a checkmark for ~3 seconds
• Verify clipboard contains the correct file path after clicking
• Test with a long file path and verify it wraps correctly

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