refactor: extract magic constants — wave 4 (UX timings + physics)

Name throttle/debounce intervals, spring presets, and layout paddings
that were inline numeric literals:

- VirtualList: VISIBLE_CHANGE_THROTTLE_MS (150), NEAR_BOTTOM_THROTTLE_MS
  (200), JUMP_THROTTLE_MS (200)
- SampleList: CHECK_POSITION_THROTTLE_MS (100)
- SliderArea: SLIDER_SPRING_CONFIG ({stiffness: 0.2, damping: 0.7}),
  SLIDER_PERSIST_DEBOUNCE_MS (100), SLIDER_PADDING_MOBILE_PX (48),
  SLIDER_PADDING_DESKTOP_PX (96)
- FontVirtualList: TOUCH_DEBOUNCE_MS (150)
- createPerspectiveManager: PERSPECTIVE_SPRING_CONFIG ({stiffness: 0.2,
  damping: 0.8})

No behavior changes — values preserved exactly.

src/widgets/ComparisonView/ui/SliderArea/SliderArea.svelte

@@ -50,6 +50,26 @@ interface Props {
 
 let { isSidebarOpen = false, class: className }: Props = $props();
 
+/**
+ * Spring tuning for the comparison slider thumb. Lower stiffness = slower
+ * follow; higher damping = less overshoot.
+ */
+const SLIDER_SPRING_CONFIG = { stiffness: 0.2, damping: 0.7 } as const;
+
+/**
+ * Debounce wait before persisting the slider position to the store.
+ * High frequency during drag → batched writes.
+ */
+const SLIDER_PERSIST_DEBOUNCE_MS = 100;
+
+/**
+ * Horizontal layout padding subtracted from container width before laying
+ * out the comparison text. Different per breakpoint to match the gutters
+ * around the slider track.
+ */
+const SLIDER_PADDING_MOBILE_PX = 48;
+const SLIDER_PADDING_DESKTOP_PX = 96;
+
 const fontA = $derived(comparisonStore.fontA);
 const fontB = $derived(comparisonStore.fontB);
 const isLoading = $derived(comparisonStore.isLoading || !comparisonStore.isReady);
@@ -89,10 +109,7 @@ $effect(() => {
     return () => observer.disconnect();
 });
 
-const sliderSpring = new Spring(50, {
-    stiffness: 0.2,
-    damping: 0.7,
-});
+const sliderSpring = new Spring(50, SLIDER_SPRING_CONFIG);
 const sliderPos = $derived(sliderSpring.current);
 
 function handleMove(e: PointerEvent) {
@@ -115,7 +132,7 @@ function startDragging(e: PointerEvent) {
 
 const storeSliderPosition = debounce((value: number) => {
     comparisonStore.sliderPosition = value;
-}, 100);
+}, SLIDER_PERSIST_DEBOUNCE_MS);
 
 $effect(() => {
     storeSliderPosition(sliderPos);
@@ -172,7 +189,7 @@ $effect(() => {
     const fontAStr = getPretextFontString(_weight, _size, fontA.name);
     const fontBStr = getPretextFontString(_weight, _size, fontB.name);
 
-    const padding = _isMobile ? 48 : 96;
+    const padding = _isMobile ? SLIDER_PADDING_MOBILE_PX : SLIDER_PADDING_DESKTOP_PX;
     const availableWidth = Math.max(0, _width - padding);
     const lineHeight = _size * _height;