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/main.tsx

@@ -9,6 +9,7 @@ import { FeedFiltersProvider } from "./components/FeedFiltersProvider";
 import { SavedViewsProvider } from "./components/SavedViewsProvider";
 import { PlaylistsProvider } from "./components/PlaylistsProvider";
 import { PlexProvider } from "./components/PlexProvider";
+import { PlexViewBridge } from "./components/PlexViewBridge";
 import { ChannelsProvider } from "./components/ChannelsProvider";
 import { FeedViewProvider } from "./components/FeedViewProvider";
 import { PrefsProvider } from "./components/PrefsProvider";
@@ -61,15 +62,17 @@ const root =
                 <SavedViewsProvider>
                   <PlaylistsProvider>
                     <PlexProvider>
-                      <ChannelsProvider>
-                        <FeedViewProvider>
-                          <PrefsProvider>
-                            <WizardProvider>
-                              <App />
-                            </WizardProvider>
-                          </PrefsProvider>
-                        </FeedViewProvider>
-                      </ChannelsProvider>
+                      <PlexViewBridge>
+                        <ChannelsProvider>
+                          <FeedViewProvider>
+                            <PrefsProvider>
+                              <WizardProvider>
+                                <App />
+                              </WizardProvider>
+                            </PrefsProvider>
+                          </FeedViewProvider>
+                        </ChannelsProvider>
+                      </PlexViewBridge>
                     </PlexProvider>
                   </PlaylistsProvider>
                 </SavedViewsProvider>