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-list-titlebar.css

@@ -22,7 +22,7 @@
   .app-list-titlebar [role="button"] {
     --app-button-padding-horizontal: 10px;
     --app-button-padding-vertical: 6px;
-    font-size: 13px;
+    font-size: var(--text-sm);
     margin: 0;
   }
 
@@ -37,7 +37,7 @@
     background: var(--app-muted-border-color);
     width: 35px;
     height: 35px;
-    font-size: 16px;
+    font-size: 16px; /* icon-font metric — intentional px */
     border-radius: var(--app-border-radius);
     display: inline-flex;
     align-items: center;