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/shared/ui/Slider/Slider.svelte

@@ -70,17 +70,17 @@ let {
 const isVertical = $derived(orientation === 'vertical');
 
 const labelClasses = `font-mono text-2xs tabular-nums shrink-0
-    text-secondary
+    text-subtle
     group-hover:text-neutral-700 dark:group-hover:text-neutral-300
     transition-colors`;
 
 const thumbClasses = `block w-2.5 h-2.5 bg-brand
-    rotate-45 shadow-sm
+    rotate-45 shadow-rest
     hover:scale-125
     focus-visible:outline-none
     focus-visible:ring-2 focus-visible:ring-brand/20
     data-active:scale-90
-    transition-transform duration-150
+    transition-transform duration-fast
     disabled:pointer-events-none disabled:opacity-50
     cursor-grab active:cursor-grabbing`;
 </script>