fix(frontend): guard copilotMode storage.get behind isClient

Missed this initializer when porting the SSR guards — copilotMode was
calling storage.get() unconditionally while its neighbours
(completedSessionIDs, isSoundEnabled, isNotificationsEnabled) already
check isClient first. Flagged by Sentry (MEDIUM): storage.get() is
designed to throw on the server, so every SSR pass was generating
noise in error tracking.

autogpt_platform/frontend/src/app/(platform)/copilot/store.ts

@@ -109,7 +109,9 @@ export const useCopilotUIStore = create<CopilotUIState>((set) => ({
   setShowNotificationDialog: (show) => set({ showNotificationDialog: show }),
 
   copilotMode:
-    storage.get(Key.COPILOT_MODE) === "fast" ? "fast" : "extended_thinking",
+    isClient && storage.get(Key.COPILOT_MODE) === "fast"
+      ? "fast"
+      : "extended_thinking",
   setCopilotMode: (mode) => {
     storage.set(Key.COPILOT_MODE, mode);
     set({ copilotMode: mode });