refactor(web): use last-seen-tick comparison for slideout listener

Replace the boolean "skip first render" ref with a ref that stores the
previously-seen tick value. The effect now compares against the stored
value and only fires when it differs, which makes the dependency
naturally used (removes the `void slideoutOpenedTick;` acknowledgement)
and self-documents the intent of the guard.

Behavior is unchanged — both forms preserve the one-shot-per-event
semantics of the prior window-event implementation. The JSDoc on
`slideoutOpenedTickAtom` is updated to describe the new pattern.

surfsense_web/lib/layout-events.ts

@@ -2,8 +2,10 @@ 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.
+ * Consumers read this with `useAtomValue` and store the last-seen value in
+ * a ref so the effect fires only when the tick changes. This preserves the
+ * one-shot semantics of the previous window-event implementation: a
+ * subscriber that mounts after a panel has already opened does not
+ * retroactively re-trigger.
  */
 export const slideoutOpenedTickAtom = atom(0);