fix: address 6 low-severity findings from typography token code review

- Replace mislabeled sidebar title 10px with var(--text-2xs) (F-001)
- Replace font-size: small with var(--text-sm) in app-shell (F-002)
- Annotate @media heading overrides as potentially redundant with clamp() (F-003)
- Correct type scale comment to "approximate major-third ratio" (F-004)
- Differentiate h5 (--text-sm) from h4 (--text-base) (F-005)
- Tighten contract test to flag bare normal/bold keywords (F-006)

Task run: UI-TYPOGRAPHY-SCALE-002

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/css/layout/app-sidebar.css

@@ -3,7 +3,7 @@
     background: var(--app-sidebar-background-color);
     --app-sidebar-gap: 0.5rem;
     --app-sidebar-item-gap: 10px;
-    --app-sidebar-title-size: 10px; /* icon-font metric — intentional px */
+    --app-sidebar-title-size: var(--text-2xs);
     --app-sidebar-summary-size: var(--text-xs);
     --app-sidebar-tenant-size: var(--text-sm);
     --app-sidebar-titlebar-height: 30px;