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/ComboControl/ComboControl.svelte

@@ -91,7 +91,7 @@ const displayLabel = $derived(label ?? controlLabel ?? '');
             step={control.step}
             orientation="horizontal"
         />
-        <span class="font-mono text-xs text-secondary tabular-nums w-10 text-right shrink-0">
+        <span class="font-mono text-xs text-subtle tabular-nums w-10 text-right shrink-0">
             {formattedValue()}
         </span>
     </div>
@@ -120,12 +120,12 @@ const displayLabel = $derived(label ?? controlLabel ?? '');
                         <button
                             {...props}
                             class={cn(
-                                'flex flex-col items-center justify-center w-14 py-1',
-                                'select-none rounded-none transition-all duration-150',
+                                'flex flex-col flex-center w-14 py-1',
+                                'select-none rounded-none transition-all duration-fast',
                                 'border border-transparent',
                                 'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-brand/30',
                                 open
-                                    ? 'bg-paper dark:bg-dark-card shadow-sm border-subtle'
+                                    ? 'surface-card-elevated'
                                     : 'hover:bg-paper/50 dark:hover:bg-dark-card/50',
                             )}
                             aria-label={controlLabel ? `${controlLabel}: ${formattedValue()}` : undefined}
@@ -134,7 +134,7 @@ const displayLabel = $derived(label ?? controlLabel ?? '');
                             {#if displayLabel}
                                 <span
                                     class="
-                                        text-3xs font-primary font-bold tracking-tight uppercase
+                                        text-3xs text-label-mono
                                         text-neutral-900 dark:text-neutral-100
                                         mb-0.5 leading-none
                                     "
@@ -153,7 +153,7 @@ const displayLabel = $derived(label ?? controlLabel ?? '');
 
                 <!-- Vertical slider popover -->
                 <Popover.Content
-                    class="w-auto py-4 px-3 h-64 flex items-center justify-center rounded-none border border-subtle shadow-sm bg-paper dark:bg-dark-card"
+                    class="w-auto py-4 px-3 h-64 flex-center rounded-none surface-card-elevated"
                     align="center"
                     side="top"
                 >