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/Sidebar/Sidebar.svelte

@@ -43,7 +43,7 @@ let {
     class={cn(
         'flex flex-col h-full',
         'w-80',
-        'bg-surface dark:bg-dark-bg',
+        'surface-canvas',
         'border-r border-subtle',
         'transition-colors duration-500',
         className,
@@ -54,7 +54,7 @@ let {
         class="
             p-6 shrink-0
             border-b border-subtle
-            bg-surface dark:bg-dark-bg
+            surface-canvas
         "
     >
         <!-- Title -->
@@ -90,7 +90,7 @@ let {
     </div>
 
     <!-- ── Main: content area (no scroll - VirtualList handles scrolling) ─────────────────────────────── -->
-    <div class="flex-1 min-h-0 bg-surface dark:bg-dark-bg">
+    <div class="flex-1 min-h-0 surface-canvas">
         {#if main}
             {@render main()}
         {/if}
@@ -101,7 +101,7 @@ let {
         <div
             class="
                 shrink-0 p-6
-                bg-surface dark:bg-dark-bg
+                surface-canvas
                 border-t border-subtle
                 z-10
             "