fix(plex): make the header Discover/Browse toggle clickable

The floating header wrapper is pointer-events-none (so clicks fall through its empty
area to the content below); every interactive child must re-enable pointer events.
PlexViewToggle rendered its SegmentedToggle without the pointer-events-auto wrapper the
sibling PlexScopeToggle has, so the whole Discover/Browse switch was click-dead — the
buttons rendered and were correctly wired, but no real mouse click ever reached them.

Add `className="pointer-events-auto shrink-0"` to the toggle and a regression test that
asserts the root re-enables pointer events (plus the render/onChange/hide-on-deeper-view
contract). Verified live on localdev: real clicks now switch Discover⟷Browse (?v=browse)
in both directions; a header-wide hit-test audit shows no other dead controls.

Gate: tsc/lint/knip/prettier clean, 418 vitest.

frontend/src/components/PlexViewToggle.tsx

@@ -11,9 +11,13 @@ export default function PlexViewToggle() {
   // Only the two title-browse views host the Discover/Browse choice; deeper views (show/info/person/…)
   // and the playlists/collections browse surfaces hide it.
   if (!ctl || (ctl.kind !== "discover" && ctl.kind !== "grid")) return null;
+  // The floating header wrapper is pointer-events-none (so clicks fall through its empty area to the
+  // content below); every interactive child must re-enable pointer events or it is click-dead — the
+  // sibling PlexScopeToggle does the same.
   return (
     <SegmentedToggle
       size="md"
+      className="pointer-events-auto shrink-0"
       ariaLabel={t("plex.view.label")}
       options={[
         { value: "discover", label: t("plex.discover.tab") },