Replace the `SLIDEOUT_PANEL_OPENED_EVENT` window event with a
`slideoutOpenedTickAtom` jotai atom. The dispatcher in
`SidebarSlideOutPanel` now bumps the tick via `useSetAtom`, and the
listener in `Thread` reads it via `useAtomValue` and reacts on change
behind a ref guard that skips the initial render — preserving the
one-shot-per-open semantics of the previous event.
This removes the implicit cross-module string contract, makes the
signal traceable through React DevTools / jotai inspector, and lets
TypeScript catch typos that the string-based event API silently
swallowed.
