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/app/styles/app.css

@@ -16,7 +16,7 @@
 
   /* Semantic mode-switching colors. These are redefined inside `.dark`
      so utilities that reference them auto-adapt without a `dark:` variant. */
-  --color-border-subtle: rgb(0 0 0 / 0.05);
+  --color-border-subtle: var(--neutral-300);
   --color-text-subtle: var(--neutral-500);
   --color-skeleton: var(--neutral-200);
 
@@ -336,6 +336,13 @@
   border-color: var(--color-border-subtle);
 }
 
+/* Same color as border-subtle, applied via background-color — for 1px
+   dividers, inline separator strips, and other hairlines that aren't
+   element borders. */
+@utility bg-subtle {
+  background-color: var(--color-border-subtle);
+}
+
 /* Muted text color — paired with `border-subtle` naming. The previous
    name `text-secondary` collided with Tailwind v4 auto-generating a
    utility from `--color-secondary` (the shadcn near-white surface token