refactor: switch icons from react-icons to Phosphor

Replace react-icons and @radix-ui/react-icons with @phosphor-icons/react
for a consistent icon style across all dashboard panels.

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

components/OfflineBanner.tsx

@@ -1,13 +1,13 @@
 'use client';
 
-import { FiWifiOff } from 'react-icons/fi';
+import { WifiSlash } from '@phosphor-icons/react';
 
 export function OfflineBanner({ isOnline }: { isOnline: boolean }) {
   if (isOnline) return null;
 
   return (
     <div className="fixed top-0 left-0 right-0 z-[100] rounded-b-xl bg-yellow-500/15 dark:bg-yellow-500/25 border-b border-yellow-500/30 dark:border-yellow-500/40 text-yellow-700 dark:text-yellow-300 px-4 sm:px-8 py-2.5 text-sm flex items-center justify-center gap-2 font-medium shadow-md transition-shadow duration-300">
-      <FiWifiOff className="w-4 h-4 shrink-0" />
+      <WifiSlash className="w-4 h-4 shrink-0" />
       <span>You are currently offline. Changes may not be saved.</span>
     </div>
   );