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/components/app-tile.css

@@ -129,7 +129,7 @@
     padding: 4px 10px 4px 10px;
     text-transform: uppercase;
     font-size: var(--text-sm);
-    letter-spacing: 0.4px;
+    letter-spacing: var(--tracking-wide);
     background: var(--app-blockquote-background-color);
   }