feat(local-library): tighter tree sidebar default + horizontal scroll

Default width drops from 432px to 302px (~30% smaller). Long folder
names no longer ellipsize - the sidebar scrolls horizontally inside
itself when content exceeds the column width. Drag-resize and the
persisted width preference are unchanged: the user can still widen
to 40% of the content area, and that choice survives restart.

The resize handle stays anchored to the column's right edge because
it sits as a sibling of the scroll wrapper (.folder-tree-scroll),
not inside it - horizontal scroll inside the wrapper does not drag
the handle along.

cc

src-tauri/src/config/library_preferences.rs

@@ -39,7 +39,7 @@ pub struct LibraryPreferences {
     #[serde(default)]
     pub folders_view_mode: FoldersViewMode,
     /// Width (in CSS pixels) of the tree-mode left sidebar in the Folders
-    /// tab. `None` means "use the frontend default" (currently 432px).
+    /// tab. `None` means "use the frontend default" (currently 302px).
     /// Persisted on drag-end so the choice survives restarts. Bounds are
     /// enforced on the frontend (min 200px, max 40% of content area).
     #[serde(default)]