chore: remove design system v3 compatibility shims

Glass-card, text-gradient-brand, active-glow, bg-gradient-brand-hover
shims removed. All components migrated to v4 flat dark theme.
font-label alias kept temporarily (still referenced in ~10 files).

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

frontend/src/index.css

@@ -73,7 +73,7 @@
   --font-sans: 'IBM Plex Sans', system-ui, -apple-system, sans-serif;
   --font-heading: 'Bricolage Grotesque', system-ui, sans-serif;
   --font-mono: 'JetBrains Mono', monospace;
-  --font-label: 'JetBrains Mono', monospace; /* deprecated alias — remove in Phase 5 */
+  --font-label: 'JetBrains Mono', monospace; /* deprecated — use font-mono for new code */
 
   /* ── Animations ────────────────────────────────── */
   --animate-fade-in: fade-in 200ms ease-out;
@@ -142,35 +142,7 @@
   }
 }
 
-/* ── Deprecated shims — remove after Phase 3 sweep ── */
+/* ── Glass/gradient shims removed — Design System v4 migration complete ── */
-@utility glass-card {
-  background: var(--color-bg-card);
-  border: 1px solid var(--color-border-default);
-  border-radius: 8px;
-  transition: border-color 200ms ease;
-  &:hover {
-    border-color: var(--color-border-hover);
-  }
-}
-
-@utility glass-card-static {
-  background: var(--color-bg-card);
-  border: 1px solid var(--color-border-default);
-  border-radius: 8px;
-}
-
-@utility text-gradient-brand {
-  color: var(--color-accent-text);
-}
-
-@utility active-glow {
-  /* glow removed in v4 — kept as no-op shim */
-  --_active-glow: 1;
-}
-
-@utility bg-gradient-brand-hover {
-  &:hover { filter: brightness(1.1); }
-}
 
 @utility stagger-item {
   opacity: 0;