fix: give Explore its own /explore route so PiP navigates back to voice correctly

The Explore page shared /channels/@me with Friends, differentiated by a
showExplore UI flag. When PiP navigated to a voice channel, the stale
flag caused MainContent to render text chat instead of the voice grid.
Replace the flag with a dedicated /explore route so the URL is the
single source of truth.

packages/web/src/App.tsx

@@ -52,6 +52,14 @@ export function App() {
           </ProtectedRoute>
         }
       />
+      <Route
+        path="/explore"
+        element={
+          <ProtectedRoute>
+            <AppLayout />
+          </ProtectedRoute>
+        }
+      />
       <Route path="/" element={<Navigate to="/channels/@me" replace />} />
       <Route path="*" element={<Navigate to="/channels/@me" replace />} />
     </Routes>