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-details.css

@@ -44,13 +44,13 @@
     display: inline-flex;
     align-items: center;
     justify-content: center;
-    font-weight: 600;
+    font-weight: var(--font-semibold);
     color: var(--app-muted-color);
   }
 
   .user-avatar-placeholder,
   .entity-avatar-placeholder {
-    font-size: 1.5rem;
+    font-size: var(--text-2xl);
   }
 
   .user-avatar-image {