fix(comparison): lay out lines to the content box, not a padding guess

availableWidth was containerWidth minus a flat 48/96px constant, but the
slider track's gutters are responsive CSS padding (px-4/8/12/lg:px-24,
per side). At lg the real padding is 192px while only 96 was subtracted,
so lines were broken ~96px too wide and overflowed the container.

Measure the container's content box (ResizeObserver contentBoxSize) and
use it directly as availableWidth; keep the border box for the slider and
split math. The content box already excludes the gutters, so it tracks the
breakpoints with no constant to maintain.

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

@@ -61,14 +61,6 @@ const SLIDER_SPRING_CONFIG = { stiffness: 0.2, damping: 0.7 } as const;
  */
 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;
-
 /**
  * Position bounds (percent of container width).
  */
@@ -94,7 +86,18 @@ const isMobile = $derived(responsive?.isMobile ?? false);
 
 let isDragging = $state(false);
 let isTypographyMenuOpen = $state(false);
+/**
+ * Border-box width of the slider track. Drives the slider→position mapping and
+ * the per-char split math (pointer events are measured against this same box).
+ */
 let containerWidth = $state(0);
+/**
+ * Content-box width of the slider track (border box minus the responsive CSS
+ * gutters). The width the text lays out into, and what the line-breaker fits
+ * within. Measured rather than derived from a padding constant so it tracks the
+ * breakpoint gutters.
+ */
+let contentWidth = $state(0);
 
 const layout = new DualFontLayout();
 
@@ -114,10 +117,14 @@ $effect(() => {
 
     const observer = new ResizeObserver(entries => {
         for (const entry of entries) {
-            // Use borderBoxSize if available, fallback to contentRect
+            // Border box for slider/pointer math, content box for text width.
-            const width = entry.borderBoxSize?.[0]?.inlineSize ?? entry.contentRect.width;
+            const border = entry.borderBoxSize?.[0]?.inlineSize ?? entry.contentRect.width;
-            if (width > 0) {
+            const content = entry.contentBoxSize?.[0]?.inlineSize ?? entry.contentRect.width;
-                containerWidth = width;
+            if (border > 0) {
+                containerWidth = border;
+            }
+            if (content > 0) {
+                contentWidth = content;
             }
         }
     });
@@ -226,7 +233,7 @@ $effect(() => {
     }
 });
 
-// Layout effect — depends on content, settings AND containerWidth.
+// Layout effect — depends on content, settings, and contentWidth.
 // Awaits font loading into the canvas measurement context before invoking
 // the engine; otherwise pretext caches fallback-font widths globally per
 // font string, and the morph boundary drifts from the thumb visually.
@@ -236,18 +243,15 @@ $effect(() => {
     const _size = typography.renderedSize;
     const _height = typography.height;
     const _spacing = typography.spacing;
-    const _width = containerWidth;
+    const availableWidth = contentWidth;
-    const _isMobile = isMobile;
 
-    if (!container || !fontA || !fontB || _width <= 0) {
+    if (!container || !fontA || !fontB || availableWidth <= 0) {
         return;
     }
 
     const fontAStr = getPretextFontString(_weight, _size, fontA.name);
     const fontBStr = getPretextFontString(_weight, _size, fontB.name);
 
-    const padding = _isMobile ? SLIDER_PADDING_MOBILE_PX : SLIDER_PADDING_DESKTOP_PX;
-    const availableWidth = Math.max(0, _width - padding);
     const lineHeight = _size * _height;
 
     let cancelled = false;