fix: preserve clipboard contents and exit cleanly

[aladin7]

What changed

• preserve safely duplicable Windows clipboard formats during dictation paste, including common image and copied-file formats
• restore the clipboard only when WritHer’s temporary transcript is still present
• retry clipboard transfers and avoid overwriting clipboard content copied during the paste
• make the clipboard restoration delay configurable
• run tray-triggered shutdown on Tk’s main thread and join tray, hotkey, and reminder threads
• cancel service startup when shutdown is requested while the Whisper model is loading

Why

WritHer previously backed up only Unicode text, so pasting dictated text could destroy screenshots, copied files, and other non-text clipboard formats. Its tray callback could also log Shutdown complete while the Tk or tray event loop still kept Python alive.

Impact

Dictation keeps the compatible Ctrl+V workflow while preserving common clipboard content. Quitting from the tray now terminates the process deterministically, including during startup.

Validation

• 54 tests pass on current upstream main
• focused Ruff checks pass
• compilation checks pass
• git diff --check passes
• native Windows tray + Tk lifecycle test exits cleanly
• native CF_DIB and CF_HDROP preservation checks passed during development