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/App.tsx

@@ -127,6 +127,7 @@ function AppShell() {
   const toggleFullscreen = usePlayerStore(s => s.toggleFullscreen);
   const isQueueVisible = usePlayerStore(s => s.isQueueVisible);
   const toggleQueue = usePlayerStore(s => s.toggleQueue);
+  const uiScale = useFontStore(s => s.uiScale);
   const initializeFromServerQueue = usePlayerStore(s => s.initializeFromServerQueue);
   const currentTrack = usePlayerStore(s => s.currentTrack);
   const isPlaying = usePlayerStore(s => s.isPlaying);
@@ -341,6 +342,7 @@ function AppShell() {
         />
       )}
       <main className="main-content">
+        <div className="main-content-zoom" style={uiScale !== 1 ? { zoom: uiScale } : undefined}>
         <header className="content-header">
           <LiveSearch />
           <div className="spacer" />
@@ -389,6 +391,7 @@ function AppShell() {
             <Route path="/device-sync" element={<DeviceSync />} />
           </Routes>
         </div>
+        </div>
       </main>
       {!isMobile && (
         <div 
@@ -919,8 +922,6 @@ export default function App() {
   useThemeStore(s => s.theme); // keep subscription so re-render on manual change
   const effectiveTheme = useThemeScheduler();
   const font = useFontStore(s => s.font);
-  const uiScale = useFontStore(s => s.uiScale);
-  const setUiScale = useFontStore(s => s.setUiScale);
   const [exportPickerOpen, setExportPickerOpen] = useState(false);
 
   useEffect(() => {
@@ -931,17 +932,10 @@ export default function App() {
     document.documentElement.setAttribute('data-font', font);
   }, [font]);
 
-  // TODO(ui-scale): UI scaling is disabled pending a cross-platform rework.
-  // Reset any stored non-100% value so users aren't stuck at a broken scale.
-  // When re-enabling: remove this effect AND re-enable the slider in Settings.tsx.
-  useEffect(() => {
-    if (uiScale !== 1.0) setUiScale(1.0);
-  // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, []);
-
-  useEffect(() => {
-    document.documentElement.style.zoom = String(uiScale);
-  }, [uiScale]);
+  // UI scaling is scoped to .main-content via an inner wrapper (see <main>
+  // below). Sidebar, queue, player bar and (Linux) custom title bar stay 1:1
+  // because they live in separate grid cells. Document-level zoom is not used
+  // — it broke portal positioning and Tauri window measurement.
 
   useEffect(() => {
     return initAudioListeners();

src/pages/Settings.tsx

@@ -1739,50 +1739,56 @@ export default function Settings() {
               <h2>{t('settings.uiScaleTitle')}</h2>
             </div>
             <div className="settings-card">
-              {/* TODO: UI scaling is being reworked — disabled until fixed */}
-              <p style={{ fontSize: 13, color: 'var(--text-secondary)', margin: 0 }}>
-                Interface scaling is currently being reworked and will be available in a future update.
-              </p>
-              <div style={{ display: 'flex', flexDirection: 'column', gap: '12px', opacity: 0.4, pointerEvents: 'none', marginTop: 12 }}>
+              <div style={{ display: 'flex', flexDirection: 'column', gap: '12px' }}>
                 <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'space-between' }}>
                   <span style={{ fontSize: 13, color: 'var(--text-secondary)' }}>{t('settings.uiScaleLabel')}</span>
                   <span style={{ fontSize: 13, fontWeight: 600, color: 'var(--accent)', minWidth: 40, textAlign: 'right' }}>
                     {Math.round(fontStore.uiScale * 100)}%
                   </span>
                 </div>
-                <input
-                  type="range"
-                  min={0.8}
-                  max={1.5}
-                  step={0.05}
-                  value={fontStore.uiScale}
-                  onChange={e => fontStore.setUiScale(parseFloat(e.target.value))}
-                  className="ui-scale-slider"
-                />
-                <div style={{ position: 'relative', height: 24 }}>
-                  {[80, 90, 100, 110, 125, 150].map(p => {
-                    const pct = ((p / 100) - 0.8) / (1.5 - 0.8) * 100;
-                    const active = Math.round(fontStore.uiScale * 100) === p;
-                    return (
-                      <button
-                        key={p}
-                        className="btn btn-ghost"
-                        style={{
-                          position: 'absolute',
-                          left: `${pct}%`,
-                          transform: 'translateX(-50%)',
-                          fontSize: 11,
-                          padding: '2px 6px',
-                          opacity: active ? 1 : 0.5,
-                          color: active ? 'var(--accent)' : undefined,
-                        }}
-                        onClick={() => fontStore.setUiScale(p / 100)}
-                      >
-                        {p}%
-                      </button>
-                    );
-                  })}
-                </div>
+                {(() => {
+                  const presets = [80, 90, 100, 110, 125, 150];
+                  const currentPct = Math.round(fontStore.uiScale * 100);
+                  let idx = presets.indexOf(currentPct);
+                  if (idx < 0) {
+                    // Snap legacy off-preset values to the closest preset.
+                    idx = presets.reduce((best, p, i) =>
+                      Math.abs(p - currentPct) < Math.abs(presets[best] - currentPct) ? i : best, 0);
+                  }
+                  return (
+                    <>
+                      <input
+                        type="range"
+                        min={0}
+                        max={presets.length - 1}
+                        step={1}
+                        value={idx}
+                        onChange={e => fontStore.setUiScale(presets[parseInt(e.target.value, 10)] / 100)}
+                        className="ui-scale-slider"
+                      />
+                      <div style={{ display: 'flex', justifyContent: 'space-between' }}>
+                        {presets.map(p => {
+                          const active = currentPct === p;
+                          return (
+                            <button
+                              key={p}
+                              className="btn btn-ghost"
+                              style={{
+                                fontSize: 11,
+                                padding: '2px 6px',
+                                opacity: active ? 1 : 0.5,
+                                color: active ? 'var(--accent)' : undefined,
+                              }}
+                              onClick={() => fontStore.setUiScale(p / 100)}
+                            >
+                              {p}%
+                            </button>
+                          );
+                        })}
+                      </div>
+                    </>
+                  );
+                })()}
               </div>
             </div>
           </section>

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;