fix: preserve images and HTML in clipboard during paste-and-restore

[egsok]

Summary

• Clipboard save/restore now handles images (screenshots), HTML (rich text from browser), and plain text — previously only plain text was preserved
• Added _saveClipboard() that detects content type via clipboard.availableFormats() and reads the appropriate format
• Added _restoreClipboard() helper that writes back the correct format after paste completes
• All 6 platform-specific restore callsites (macOS CGEvent, macOS osascript, Windows fast-paste, Windows nircmd, Windows PowerShell, Linux) updated to use the new helper

Problem

If a user had a screenshot (Win+Shift+S) or rich HTML in the clipboard before dictation, readText() returned "", and after paste writeText("") overwrote the image/HTML with nothing.

Test plan

• Copy a screenshot to clipboard → dictate → verify screenshot restores (Ctrl+V pastes the image)
• Copy plain text to clipboard → dictate → verify text restores
• Copy rich HTML from browser → dictate → verify HTML restores when pasting into a rich text editor
• Verify dictated text still gets pasted correctly into the active app

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