refactor: sweep call sites onto design-system utilities + bug fixes

Replace inline class clusters with the design-system utilities and
tokens established in the prior two commits. No behavior changes
intended beyond two real bug fixes.

Bug fixes:
- SampleList.svelte: 'border-border-subtle bg-background-40' was a
  silent no-op (both classes mis-spelled). Now 'border-subtle
  bg-background/40' applies as intended.
- FontList.svelte: 'h-[44px]' → 'h-11' (44px = 2.75rem = spacing-11,
  no need for arbitrary value).

Sweeps:
- TypographyMenu: popover + floating bar now use surface-popover /
  surface-floating + shadow-popover.
- FontList + FilterGroup: tertiary list buttons use the new
  Button layout="block-list-row" variant; skeleton fills use
  the skeleton-fill utility.
- Footer / BreadcrumbHeader: surface-floating absorbs the
  bg-surface/blur/border cluster. Footer bumped to z-20 with a
  comment explaining the stacking against SidebarContainer (z-40/50).
- FontSampler: surface-card + hover shadow-stamp-card token.
- SliderArea: surface-canvas, flex-center, shadow-floating-panel
  tokens (light + dark variants).
- Sidebar / Header / ButtonGroup / Layout / SidebarContainer:
  bg-surface dark:bg-dark-bg → surface-canvas (8 sites);
  SidebarContainer mobile panel uses shadow-overlay.
- Loader / Thumb: flex items-center justify-center → flex-center;
  Thumb durations → duration-fast.
- ComboControl: trigger uses surface-card-elevated when open,
  popover uses surface-card-elevated, label cluster → text-label-mono,
  flex-center for the trigger interior.
- Slider: shadow-sm → shadow-rest, duration-150 → duration-fast.
- text-secondary → text-subtle across Input, Slider, ComboControl
  (matches the rename in the styles commit).
- Link: reverted earlier surface-floating attempt — Link's original
  bg-surface/80 backdrop-blur pattern was thinner than surface-floating
  (no border, smaller blur), and the Footer was overlaying its own
  border-subtle on top, fighting the utility. Kept the original style.

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

@@ -233,14 +233,14 @@ const scaleClass = $derived(
     Outer flex container — fills parent.
     The paper div inside scales down when the sidebar opens on desktop.
 -->
-<div class={cn('flex-1 relative flex items-center justify-center p-0 overflow-hidden bg-surface dark:bg-dark-bg', className)}>
+<div class={cn('flex-1 relative flex-center p-0 overflow-hidden surface-canvas', className)}>
     <!-- Paper surface -->
     <div
         class={cn(
-            'w-full h-full flex flex-col items-center justify-center relative',
+            'w-full h-full flex flex-col flex-center relative',
             'bg-paper dark:bg-dark-card',
-            'shadow-2xl shadow-black/5 dark:shadow-black/20',
-            'transition-transform duration-300 ease-out',
+            'shadow-floating-panel dark:shadow-floating-panel-dark',
+            'transition-transform duration-slow ease-out',
             scaleClass,
         )}
     >