refactor(web): replace slideout panel window event with jotai atom (fixes #1358)

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.

surfsense_web/lib/layout-events.ts

@@ -1 +1,9 @@
-export const SLIDEOUT_PANEL_OPENED_EVENT = "slideout-panel-opened";
+import { atom } from "jotai";
+
+/**
+ * Tick counter that increments each time a sidebar slide-out panel opens.
+ * Consumers read this with `useAtomValue` and react to it changing — guard
+ * the initial render with a ref so the effect only fires on subsequent
+ * opens, matching the one-shot semantics of the previous window event.
+ */
+export const slideoutOpenedTickAtom = atom(0);