feat: extract typography tokens into dedicated file with semantic aliases and a11y fallbacks

Create typography.tokens.css as single source of truth for the type system:
- Extract all primitive tokens from variables.base.css (--text-*, --leading-*, --font-*, --tracking-*)
- Add semantic aliases (--text-body, --text-label, --text-caption, --text-title, --text-page-title)
- Add --text-md (15px) escape hatch and --tracking-wider (0.05em) token
- Add prefers-reduced-motion fallbacks locking fluid headings to fixed midpoints
- Document scale ratio (1.25x major third, 14px base) with reference table
- Migrate h1/h2 in app-shell.css to semantic aliases
- Migrate 4 raw letter-spacing values to --tracking-* tokens
- Extend TypographyTokenContractTest with 3 new tests (letter-spacing, aliases, reduced-motion)

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

web/css/layout/app-shell.css

@@ -36,13 +36,13 @@
   }
 
   h1 {
-    --app-font-size: var(--text-2xl);
+    --app-font-size: var(--text-page-title);
     --app-line-height: var(--leading-tight);
     --app-typography-spacing-top: 3rem;
   }
 
   h2 {
-    --app-font-size: var(--text-xl);
+    --app-font-size: var(--text-title);
     --app-line-height: var(--leading-tight);
     --app-typography-spacing-top: 2.625rem;
   }
@@ -178,7 +178,7 @@
   th {
     text-transform: uppercase;
     font-size: var(--text-xs);
-    letter-spacing: 0.4px;
+    letter-spacing: var(--tracking-wide);
     border-top: 1px solid var(--app-border);
   }