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/HandoffModal.tsx

@@ -156,7 +156,7 @@ export function HandoffModal({ onClose, onSubmit }: HandoffModalProps) {
             type="button"
             onClick={handleSubmit}
             disabled={isSubmitting}
-            className="rounded-[5px] bg-accent px-4 py-2 text-sm font-medium text-white hover:bg-[#ea580c] transition-colors disabled:opacity-50"
+            className="rounded-[5px] bg-accent px-4 py-2 text-sm font-medium text-white hover:bg-accent-hover transition-colors disabled:opacity-50"
           >
             {isSubmitting ? 'Submitting…' : intent === 'park' ? 'Park Session' : 'Escalate Session'}
           </button>