feat(linux): add WebKitGTK smooth wheel scroll toggle in settings

Persist preference in auth store, sync from App on Linux, and expose a
Tauri command using webkit2gtk to toggle enable-smooth-scrolling at
runtime. Default on to match upstream; users may disable for discrete
GTK-style line steps.

Apply a one-time rehydrate migration so smooth scrolling stays on after
updates even if an older build persisted the wrong default.

src/App.tsx

@@ -139,6 +139,7 @@ function AppShell() {
   const activeServerId = useAuthStore(s => s.activeServerId);
   const setMusicFolders = useAuthStore(s => s.setMusicFolders);
   const useCustomTitlebar = useAuthStore(s => s.useCustomTitlebar);
+  const linuxWebkitKineticScroll = useAuthStore(s => s.linuxWebkitKineticScroll);
   const setEntityRatingSupport = useAuthStore(s => s.setEntityRatingSupport);
   const offlineAlbums = useOfflineStore(s => s.albums);
   const hasOfflineContent = Object.values(offlineAlbums).some(a => a.serverId === serverId);
@@ -151,6 +152,11 @@ function AppShell() {
     invoke('set_window_decorations', { enabled }).catch(() => {});
   }, [useCustomTitlebar, isTilingWm]);
 
+  useEffect(() => {
+    if (!IS_LINUX) return;
+    invoke('set_linux_webkit_smooth_scrolling', { enabled: linuxWebkitKineticScroll }).catch(() => {});
+  }, [linuxWebkitKineticScroll]);
+
   useEffect(() => {
     if (!isLoggedIn || !activeServerId) return;
     const serverAtStart = activeServerId;