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/base/variables.base.css

@@ -75,7 +75,7 @@
   --app-empty-text: var(--app-color);
   --app-empty-hint: var(--app-muted-color);
 
-  /* ── Typography scale (major-third 1.25 ratio) ──────────────── */
+  /* ── Typography scale (approximate major-third ratio 1.25×, tighter steps below base) ── */
   --text-2xs: 0.625rem;
   --text-xs: 0.75rem;
   --text-sm: 0.875rem;