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/Label/config.ts

@@ -25,7 +25,9 @@ export const labelSizeConfig: Record<LabelSize, string> = {
 export const labelVariantConfig: Record<LabelVariant, string> = {
     default: 'text-neutral-900 dark:text-neutral-100',
     accent: 'text-brand',
-    muted: 'text-neutral-400 dark:text-neutral-500',
+    /* Light mode bumped from neutral-400 (~2.7:1 contrast, barely visible)
+       to neutral-600 (~7.5:1). Dark mode unchanged. */
+    muted: 'text-neutral-600 dark:text-neutral-500',
     success: 'text-green-600 dark:text-green-400',
     warning: 'text-yellow-600 dark:text-yellow-400',
     error: 'text-brand',