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/Header.tsx

@@ -5,6 +5,7 @@ import type { Me } from "../lib/api";
 import SearchBar from "./SearchBar";
 import PlexSearchBox from "./PlexSearchBox";
 import PlexScopeToggle from "./PlexScopeToggle";
+import PlexViewToggle from "./PlexViewToggle";
 import FeedScopeToggle from "./FeedScopeToggle";
 import SyncStatus from "./SyncStatus";
 import { useNavigation } from "./NavigationProvider";
@@ -103,8 +104,10 @@ const Header = forwardRef<
           so the primary "what am I browsing" control is always in reach (mirrors Plex's scope toggle). */}
       {page === "feed" && <FeedScopeToggle />}
       {/* Library scope (movie/show/both) sits right of the search box on Plex — relocated here from
-          the filter sidebar (S3) so the primary "what am I browsing" control is always in reach. */}
+          the filter sidebar (S3) so the primary "what am I browsing" control is always in reach. The
+          Discover ⟷ Browse toggle sits right beside it (driven via the PlexViewBridge). */}
       {page === "plex" && me.plex_enabled && <PlexScopeToggle />}
+      {page === "plex" && me.plex_enabled && <PlexViewToggle />}
       {/* Per-user sync status — it used to cost the nav rail a whole block. It sits with the other
           pills rather than pinned to the far right: on a wide window that just strands it. Only on
           the two modules it says anything about: the feed those subscriptions fill, and the manager