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/Button/Button.svelte

@@ -143,9 +143,11 @@ const variantStyles: Record<ButtonVariant, string> = {
     tertiary: cn(
         // Font override — must come after base in cn() to win via tailwind-merge
         'font-secondary font-medium normal-case tracking-normal',
-        // Inactive state
+        // Inactive state — bumped in light mode for readable contrast against
+        // bg-surface (~7.5:1 vs. the prior ~2.7:1 with neutral-400). Dark
+        // unchanged because the existing tone reads well on dark-bg.
         'bg-transparent',
-        'text-neutral-400 dark:text-neutral-400',
+        'text-neutral-600 dark:text-neutral-400',
         'border border-transparent',
         // Hover (inactive) — semi-transparent lift, no bg-paper token
         'hover:bg-paper/50 dark:hover:bg-dark-card/50',

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,
     )}

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',

src/shared/ui/Link/Link.svelte

@@ -34,7 +34,7 @@ let {
 <a
     class={cn(
         'group inline-flex items-center gap-1 text-2xs font-mono uppercase tracking-wider-mono',
-        'text-neutral-400 hover:text-brand transition-colors',
+        'text-neutral-500 dark:text-neutral-400 hover:text-brand transition-colors',
         'bg-surface/80 dark:bg-dark-bg/80 backdrop-blur-sm px-2 py-1 pointer-events-auto',
         className,
     )}