fix: replace hardcoded colors with CSS variables in branching components

All semantic colors now use design system tokens:
- #34d399 → text-success / bg-success-dim
- #f87171 → text-danger / bg-danger-dim
- #eab308 → text-warning / bg-warning-dim
- yellow-400 → text-warning / bg-warning-dim / border-warning
- #ea580c → hover:bg-accent-hover
- red-400 → text-danger / bg-danger-dim

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

frontend/src/components/session/BranchNode.tsx

@@ -20,26 +20,26 @@ const STATUS_CONFIG: Record<BranchStatus, StatusConfig> = {
   },
   solved: {
     icon: CheckCircle2,
-    textClass: 'text-[#34d399]',
-    badgeClass: 'bg-[rgba(52,211,153,0.10)] text-[#34d399]',
+    textClass: 'text-success',
+    badgeClass: 'bg-success-dim text-success',
     label: 'Solved',
   },
   dead_end: {
     icon: XCircle,
-    textClass: 'text-[#f87171]',
-    badgeClass: 'bg-[rgba(248,113,113,0.10)] text-[#f87171]',
+    textClass: 'text-danger',
+    badgeClass: 'bg-danger-dim text-danger',
     label: 'Dead End',
   },
   untried: {
     icon: Circle,
     textClass: 'text-muted',
-    badgeClass: 'bg-[rgba(79,86,102,0.20)] text-muted',
+    badgeClass: 'bg-elevated text-muted',
     label: 'Untried',
   },
   revived: {
     icon: RotateCcw,
-    textClass: 'text-[#eab308]',
-    badgeClass: 'bg-[rgba(234,179,8,0.10)] text-[#eab308]',
+    textClass: 'text-warning',
+    badgeClass: 'bg-warning-dim text-warning',
     label: 'Revived',
   },
 }