feat(ui-scale): re-enable scoped interface scaling

Document-level zoom broke portal positioning and Tauri window measurement,
so the slider had been forced to 100 %. Scope the zoom to a wrapper inside
.main-content instead — sidebar, queue, player bar and the Linux custom
title bar live in sibling grid cells of .app-shell and stay 1:1.

- App.tsx: drop the document-level zoom + auto-reset; wrap main-content
  children in <div class="main-content-zoom" style={zoom: uiScale}>.
- layout.css: add .main-content-zoom (flex column, fills parent).
- Settings.tsx: re-enable the slider and snap it to the 6 preset stops
  (80/90/100/110/125/150) so the thumb lands directly under each preset
  button. Legacy off-preset values snap to the nearest stop on load.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/styles/layout.css

@@ -940,6 +940,20 @@
   z-index: 1;
 }
 
+/* Scoped UI scaling target. Sidebar / queue / player / titlebar live in
+   sibling grid cells of .app-shell and stay at 1:1. The wrapper inherits
+   .main-content's flex column layout so .content-header (fixed height) and
+   .content-body (flex: 1, scroll) keep working unchanged.
+   Zoom is applied via inline style only when uiScale !== 1, so the default
+   case has no extra layer or layout cost. */
+.main-content-zoom {
+  display: flex;
+  flex-direction: column;
+  flex: 1;
+  min-height: 0;
+  min-width: 0;
+}
+
 .content-header {
   display: flex;
   align-items: center;