feat(ui): add fade transition for app switching

- Add AnimatePresence + motion.div wrapper for provider list
- Use key={activeApp} to trigger enter/exit animations on app switch
- Remove redundant animate-slide-up from ProviderList to prevent
  animation conflicts and visual jitter

src/components/providers/ProviderList.tsx

@@ -83,10 +83,7 @@ export function ProviderList({
         items={sortedProviders.map((provider) => provider.id)}
         strategy={verticalListSortingStrategy}
       >
-        <div
-          className="space-y-3 animate-slide-up"
-          style={{ animationDelay: "0.1s" }}
-        >
+        <div className="space-y-3">
           {sortedProviders.map((provider) => (
             <SortableProviderCard
               key={provider.id}