refactor(styles): improve light-mode contrast across surfaces and muted text

Dark mode unchanged. Targets that were reported as "barely visible" in
light theme:

Surfaces / dividers
- --color-border-subtle (light) bumped from rgb(0 0 0 / 0.05) to
  --neutral-300 (matches the Input underline variant's border color and
  yields a visible hairline on bg-surface / bg-paper).
- New bg-subtle utility (same color as border-subtle but as
  background-color) — used by Divider component and the TypographyMenu
  inline column separator. Replaces ad-hoc 'bg-black/5 dark:bg-white/10'
  and 'bg-black/10 dark:bg-white/10' bands.
- FontSearch + ComparisonView Search wrapper borders switched from
  hand-written 'border-swiss-black/5 dark:border-white/10' to
  border-subtle so they participate in the palette.

Muted text
- Button tertiary inactive text (light) bumped neutral-400 → neutral-600
  (~2.7:1 → ~7.5:1 contrast). Covers the A/B toggle and the font-list
  rows in the sidebar.
- Label/TechText muted variant (light) bumped neutral-400 → neutral-600.
  Covers the ComboControl value text.
- Link text aligned to neutral-500 / neutral-400 (subtle but visible).

No behavior changes; pure styling.

src/shared/ui/Divider/Divider.svelte

@@ -25,7 +25,7 @@ let {
 
 <div
     class={cn(
-        'bg-black/10 dark:bg-white/10',
+        'bg-subtle',
         orientation === 'horizontal' ? 'w-full h-px' : 'w-px h-full',
         className,
     )}