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/shared/ui/VirtualList/VirtualList.svelte

@@ -167,17 +167,33 @@ $effect(() => {
     }
 });
 
+/**
+ * Throttle for visible-items change callbacks. Lower = more responsive
+ * downstream UI; higher = fewer recomputes during scroll.
+ */
+const VISIBLE_CHANGE_THROTTLE_MS = 150;
+
+/**
+ * Throttle for near-bottom callbacks (typically used to prefetch next page).
+ */
+const NEAR_BOTTOM_THROTTLE_MS = 200;
+
+/**
+ * Throttle for jump callbacks (programmatic scroll-to-index).
+ */
+const JUMP_THROTTLE_MS = 200;
+
 const throttledVisibleChange = throttle((visibleItems: T[]) => {
     onVisibleItemsChange?.(visibleItems);
-}, 150); // 150ms throttle
+}, VISIBLE_CHANGE_THROTTLE_MS);
 
 const throttledNearBottom = throttle((lastVisibleIndex: number) => {
     onNearBottom?.(lastVisibleIndex);
-}, 200); // 200ms throttle
+}, NEAR_BOTTOM_THROTTLE_MS);
 
 const throttledOnJump = throttle((targetIndex: number) => {
     onJump?.(targetIndex);
-}, 200);
+}, JUMP_THROTTLE_MS);
 
 // Calculate top/bottom padding for spacer elements
 // In CSS Grid, gap creates space BETWEEN elements.