fix: parent-directory keys, exit sound, and upload overwrite prompt

[Nick6489]

Summary

• Backspace, Alt+Left, and Alt+Up in a file pane did nothing. Those keys were bound to the list's own key event, which wxDragon never reports a key code for, so the handler never matched them. They now use the real key-down (and character) events. Ctrl+Up and Ctrl+[ do the same, which is Command+Up and Command+[ on a Mac.
• The exit sound was started and then the process died, which tears down the audio device mid-chime. Closing now waits for that sound to finish, with a short cap so a hung device cannot trap the app.
• Uploading always asked whether to replace the remote file, even when nothing of that name was there. The prompt now appears only when the remote listing already has that name, matching downloads.

Test plan

• cargo test -p portkeydrop-core --lib -- local_files::
• cargo test -p portkeydrop --lib -- keys:: ids:: view:: file_pane::
• cargo clippy -p portkeydrop-core -p portkeydrop --all-targets -- -D warnings
• In a file pane, Backspace / Alt+Left / Alt+Up (Command+Up / Command+[ on a Mac) go to the parent directory; unmodified arrows still move the cursor; those keys still edit text in the path bar
• Quit with the exit sound enabled and hear it finish before the process goes away
• Upload a new file with nothing of that name on the server: no replace prompt. Upload the same name again: the prompt appears