feat(theme-selector): remember the imported/saved file so save overwrites it

Import now goes through showOpenFilePicker and keeps the returned file handle, and the rename handler no longer drops the handle on every keystroke. Once a theme has been imported or saved, save writes back to that same file (the first write asks once for permission, then overwrites silently); the save button title reflects whether it will overwrite or prompt for a location. Browsers without the File System Access API fall back to the hidden file input and a download-style save, as before.

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