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-tabs.css

@@ -40,8 +40,8 @@
     border-radius: var(--app-border-radius);
     background-color: transparent;
     color: var(--app-secondary);
-    font-size: 13px;
-    line-height: 1;
+    font-size: var(--text-sm);
+    line-height: var(--leading-none);
     cursor: pointer;
     z-index: 1;
     margin-block: auto;