fix(ui): overlay scrollbars, resizer hit-test, and Linux mini wheel (#255)

Add OverlayScrollArea with shared thumb metrics and drag handling; size the
thumb against the rail track height so panel insets cannot push it past the
visible rail. Route scroll uses a stable viewport id; Genres restores scroll
and infinite-scroll observation against that viewport (merged with upstream
virtualized genres list and lazy routes behind Suspense).

Suppress queue resizer activation when the pointer targets the main-route
overlay scrollbar; disable the resizer while dragging the thumb and use a
grabbing cursor on the body.

Apply WebKitGTK smooth wheel to the mini webview as well as main; the mini
window reapplies the persisted setting after auth store hydration.

src/components/Sidebar.tsx

@@ -19,6 +19,7 @@ import WhatsNewBanner from './WhatsNewBanner';
 import { getPlaylists } from '../api/subsonic';
 import { usePlaylistStore } from '../store/playlistStore';
 import { ALL_NAV_ITEMS } from '../config/navItems';
+import OverlayScrollArea from './OverlayScrollArea';
 
 
 export default function Sidebar({
@@ -155,6 +156,24 @@ export default function Sidebar({
       </button>
 
       <nav className="sidebar-nav" aria-label="Main navigation">
+        <OverlayScrollArea
+          className="sidebar-nav-scroll"
+          viewportClassName="sidebar-nav-viewport"
+          railInset="panel"
+          measureDeps={[
+            isCollapsed,
+            playlistsExpanded,
+            playlists.length,
+            isLoggedIn,
+            randomNavMode,
+            filterId,
+            hasOfflineContent,
+            activeJobs.length,
+            isSyncing,
+            syncJobTotal,
+            sidebarItems.length,
+          ]}
+        >
         {!isCollapsed && (showLibraryPicker ? (
           <>
             <button
@@ -384,6 +403,7 @@ export default function Sidebar({
             )}
           </div>
         )}
+        </OverlayScrollArea>
       </nav>
     </aside>
   );