`render_settings_item_link` was calling `cx.read_from_clipboard()`
during render so it could show a check icon next to the copy-link button
when the matching link was on the clipboard. This had two problems:

- A clipboard read per visible setting per frame is too expensive.
- On Windows, reading the clipboard pumps the system message queue. If a
queued message handler updates `App` while we're still rendering, GPUI
panics with `RefCell already borrowed` (many occurrences observed).

Track the `json_path` of the most recently copied setting locally
instead. The check icon now reflects what was copied in this session via
this UI rather than whatever is on the system clipboard.

While this removes the most common offender, the underlying
`gpui_windows` reentrancy bug still exists: `on_close` /
`on_request_frame` callbacks can be invoked while `App` is already
borrowed on Windows, and can be triggered by any other
clipboard-touching code path. We should consider a follow-up PR that
handles this at the platform layer -- either by deferring callbacks that
re-borrow `App`, or by guarding individual handlers in
`gpui_windows::events` against reentrant `borrow_mut` calls.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed a crash on Windows that could occur when closing the settings
window
- Improved the overall performance of the settings window