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/pages/DevBranchingPage.tsx

@@ -364,7 +364,7 @@ function MockResolutionPanel({ outputs }: { outputs: ResolutionOutputResponse[]
             <button
               type="button"
               onClick={() => { toast.success('Saved (mock)'); setIsEditing(false) }}
-              className="rounded-[5px] bg-accent px-3 py-1.5 text-xs font-medium text-white hover:bg-[#ea580c] transition-colors"
+              className="rounded-[5px] bg-accent px-3 py-1.5 text-xs font-medium text-white hover:bg-accent-hover transition-colors"
             >
               Save
             </button>
@@ -392,7 +392,7 @@ function MockResolutionPanel({ outputs }: { outputs: ResolutionOutputResponse[]
               disabled={!activeOutput}
               className={cn(
                 'flex items-center gap-1.5 rounded-[5px] border border-default px-3 py-1.5 text-xs transition-colors disabled:opacity-40',
-                copied ? 'border-[#34d399] text-[#34d399]' : 'text-secondary hover:bg-elevated hover:text-primary'
+                copied ? 'border-success text-success' : 'text-secondary hover:bg-elevated hover:text-primary'
               )}
             >
               {copied ? <Check size={12} /> : <Copy size={12} />}
@@ -501,7 +501,7 @@ export default function DevBranchingPage() {
             <button
               type="button"
               onClick={() => setShowHandoff(true)}
-              className="rounded-[5px] bg-accent px-4 py-2 text-sm font-medium text-white hover:bg-[#ea580c] transition-colors"
+              className="rounded-[5px] bg-accent px-4 py-2 text-sm font-medium text-white hover:bg-accent-hover transition-colors"
             >
               Open Handoff Modal
             </button>