feat: introduce mathematical type scale with single-source-of-truth typography tokens

Define a major-third (1.25) type scale as CSS custom properties in the tokens
layer (--text-2xs through --text-5xl), along with line-height (--leading-*),
font-weight (--font-*), and letter-spacing (--tracking-*) tokens. Migrate all
28 core CSS files from ad-hoc values to token references with clamp()-based
responsive scaling for headings. Add TypographyTokenContractTest to enforce
token-only usage via grep-based assertions.

Task run: UI-TYPOGRAPHY-SCALE-001

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/css/pages/app-error.css

@@ -9,15 +9,15 @@
 }
 
 .error-page__code {
-  font-size: 5rem;
-  font-weight: 700;
-  line-height: 1;
+  font-size: var(--text-5xl);
+  font-weight: var(--font-bold);
+  line-height: var(--leading-none);
   color: var(--color-text-muted);
 }
 
 .error-page__message {
   margin-top: 1rem;
-  font-size: 1.125rem;
+  font-size: var(--text-lg);
   color: var(--color-text);
 }
 
@@ -25,7 +25,7 @@
   display: inline-block;
   margin-top: 2rem;
   padding: 0.5rem 1.25rem;
-  font-size: 0.875rem;
+  font-size: var(--text-sm);
   text-decoration: none;
   border: 1px solid var(--color-border);
   border-radius: 0.375rem;