refactor: remove canUseFeature from useSubscription

No external call sites existed. Feature gating now handled by
useFeatureFlag hook backed by the feature_flags system.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/hooks/useSubscription.ts

@@ -10,11 +10,6 @@ export function useSubscription() {
 
   const isPaidPlan = plan === 'pro' || plan === 'team'
 
-  const canUseFeature = (feature: 'custom_branding' | 'priority_support'): boolean => {
-    if (!limits) return false
-    return limits[feature]
-  }
-
   const isAtTreeLimit = (): boolean => {
     if (!limits || !usage) return false
     if (limits.max_trees === null) return false // unlimited
@@ -37,7 +32,6 @@ export function useSubscription() {
     usage,
     isActive,
     isPaidPlan,
-    canUseFeature,
     isAtTreeLimit,
     isAtSessionLimit,
     formatLimit,