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/app-layers.css

@@ -12,6 +12,9 @@
  */
 @layer tokens, vendor, layout, vendor-overrides, components, pages, utilities;
 
+/* Load typography tokens first — consumed by variables.base.css and all downstream layers. */
+@import url("base/typography.tokens.css") layer(tokens);
+
 /* Load third-party styles into the vendor layer so app layers can override them predictably. */
 @import url("../vendor/bootstrap-icons/bootstrap-icons.css") layer(vendor);
 @import url("../vendor/gridjs/mermaid.min.css") layer(vendor);