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/base/variables.base.css

@@ -74,6 +74,37 @@
   --app-empty-bg: var(--app-card-background-color);
   --app-empty-text: var(--app-color);
   --app-empty-hint: var(--app-muted-color);
+
+  /* ── Typography scale (major-third 1.25 ratio) ──────────────── */
+  --text-2xs: 0.625rem;
+  --text-xs: 0.75rem;
+  --text-sm: 0.875rem;
+  --text-base: 1rem;
+  --text-lg: 1.125rem;
+  --text-xl: 1.25rem;
+  --text-2xl: clamp(1.25rem, 1.1rem + 0.5vw, 1.5rem);
+  --text-3xl: clamp(1.5rem, 1.3rem + 0.75vw, 1.875rem);
+  --text-4xl: clamp(1.75rem, 1.5rem + 1vw, 2.25rem);
+  --text-5xl: clamp(2.25rem, 1.9rem + 1.5vw, 3rem);
+
+  /* ── Line-height tokens ─────────────────────────────────────── */
+  --leading-none: 1;
+  --leading-tight: 1.2;
+  --leading-snug: 1.375;
+  --leading-normal: 1.5;
+  --leading-relaxed: 1.625;
+  --leading-loose: 1.75;
+
+  /* ── Font-weight tokens ─────────────────────────────────────── */
+  --font-regular: 400;
+  --font-medium: 500;
+  --font-semibold: 600;
+  --font-bold: 700;
+
+  /* ── Letter-spacing tokens ──────────────────────────────────── */
+  --tracking-tight: -0.01em;
+  --tracking-normal: 0;
+  --tracking-wide: 0.025em;
 }
 
 /* Icons */