Emit `tauri://close-requested` to JS, closes #2996

Editor assessment

The subject states the feature precisely: the tauri://close-requested event is emitted to JS, with a meaningful core scope and a well-formed closes #2996 footer. The event, its channel syntax, the receiving surface, and the traceability are all present, and the implied motivation (letting scripts intercept close requests) is self-evident, so no body is required. The feat type is right, no additional footers appear, and hygiene is clean, with the event name quoted exactly as it will appear in code. A compact, complete message about a new lifecycle hook.