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-flash.css

@@ -60,8 +60,8 @@
     position: relative;
     overflow: hidden;
     color: var(--app-notice-border-color);
-    font-size: 0.875rem;
-    line-height: 1.4;
+    font-size: var(--text-sm);
+    line-height: var(--leading-snug);
     pointer-events: auto;
     margin: 0;
     animation: toast-slide-in 250ms ease-out;
@@ -159,7 +159,7 @@
     opacity: 0.6;
     cursor: pointer;
     padding: 2px 4px;
-    line-height: 1;
+    line-height: var(--leading-none);
     flex-shrink: 0;
   }