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/components/app-breadcrumb.css

@@ -5,7 +5,7 @@
   display: flex;
   align-items: center;
   gap: 3px;
-  font-size: 11px;
+  font-size: var(--text-xs);
 }
 
 .app-breadcrumb a {