fix: AdminUserRestrictionsPage - use same tier limits fallback as TierLimitsPage

Use `null` (unlimited) instead of `feature.default_limit` when no
tier_limits entry exists. This fixes Selfhosted tier showing 0
instead of ∞ for features like AI analysis.

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

frontend/src/pages/AdminUserRestrictionsPage.jsx

@@ -62,7 +62,8 @@ export default function AdminUserRestrictionsPage() {
       const limits = {}
       features.forEach(feature => {
         const key = `${userTier}:${feature.id}`
-        limits[feature.id] = limitsMatrix.limits[key] ?? feature.default_limit
+        // Use same fallback logic as TierLimitsPage: undefined → null (unlimited)
+        limits[feature.id] = limitsMatrix.limits[key] ?? null
       })
       setTierLimits(limits)