feat(plex): move the Discover/Browse toggle into the header

The Discover ⟷ Browse switch now sits in the header next to the library-scope
pill (All / Movies / Shows), instead of on the chip row / atop the Discover
surface.

It's driven through a small one-way PlexViewBridge: PlexBrowse still OWNS the
sub-view (useHistorySubview) and publishes its top-level view + opener to the
bridge; the always-mounted Header reads it to render PlexViewToggle. A global
sub-view provider was deliberately avoided — PlexBrowse's hook remounts on every
plex entry and re-seeds from history (a page switch clears ?v + drops _sub), so
each entry starts at the Discover root; lifting it out would let the view go
stale across navigation. The toggle hides on deeper views (show/info/person/…)
and on non-plex pages (bridge cleared on unmount). sortView is just SortControl
now; the grid toolbar no longer carries the toggle.

Verified: toggle in the header, switches discover⟷grid, hides on info + on Feed,
and Feed→Plex resets to Discover (no stale view); no console errors.

frontend/src/components/PlexViewToggle.tsx

@@ -0,0 +1,24 @@
+import { useTranslation } from "react-i18next";
+import SegmentedToggle from "./SegmentedToggle";
+import { usePlexViewControls } from "./PlexViewBridge";
+
+// The Discover ⟷ Browse switch, hosted in the header next to the library-scope pill (All/Movies/Shows).
+// It reflects + drives PlexBrowse's top-level sub-view through the PlexViewBridge, and hides itself on
+// the deeper views (a show/info/person page has no Discover/Browse choice).
+export default function PlexViewToggle() {
+  const { t } = useTranslation();
+  const ctl = usePlexViewControls();
+  if (!ctl || !ctl.toggleable) return null;
+  return (
+    <SegmentedToggle
+      size="md"
+      ariaLabel={t("plex.view.label")}
+      options={[
+        { value: "discover", label: t("plex.discover.tab") },
+        { value: "grid", label: t("plex.view.browse") },
+      ]}
+      value={ctl.isDiscover ? "discover" : "grid"}
+      onChange={(v) => ctl.open(v === "discover" ? { kind: "discover" } : { kind: "grid" })}
+    />
+  );
+}