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/layout/app-sidebar.css

@@ -3,9 +3,9 @@
     background: var(--app-sidebar-background-color);
     --app-sidebar-gap: 0.5rem;
     --app-sidebar-item-gap: 10px;
-    --app-sidebar-title-size: 10px;
-    --app-sidebar-summary-size: 12px;
-    --app-sidebar-tenant-size: 14px;
+    --app-sidebar-title-size: 10px; /* icon-font metric — intentional px */
+    --app-sidebar-summary-size: var(--text-xs);
+    --app-sidebar-tenant-size: var(--text-sm);
     --app-sidebar-titlebar-height: 30px;
     --app-sidebar-border-width: 2px;
     --app-sidebar-summary-max-width: 142px;
@@ -37,7 +37,7 @@
   }
 
   .app-sidebar-group.app-sidebar-admin-group details summary i {
-    font-size: 10px;
+    font-size: 10px; /* icon-font metric — intentional px */
     position: relative;
     top: -4px;
     margin-right: 3px;
@@ -100,8 +100,8 @@
   }
 
   .app-sidebar-tenant-name {
-    font-weight: 600;
-    font-size: 17px;
+    font-weight: var(--font-semibold);
+    font-size: var(--text-base);
     color: var(--app-contrast);
     text-align: left;
     overflow: hidden;
@@ -126,7 +126,7 @@
     background: transparent;
     color: var(--app-muted-color);
     padding: 0.25rem;
-    font-size: 0.9rem;
+    font-size: var(--text-sm);
     cursor: pointer;
     margin-bottom: 0;
   }