fix(frontend): make chat spinner centred when loading (#12154)

## Changes 🏗️

<img width="800" height="969" alt="Screenshot 2026-02-18 at 20 30 36"
src="https://github.com/user-attachments/assets/30d7d211-98c1-4159-94e1-86e81e29ad43"
/>

- Make the spinner centred when the chat is loading

## Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Run the app and test locally

Author: 0ubbe

autogpt_platform/frontend/src/app/(platform)/copilot/components/ChatMessagesContainer/ChatMessagesContainer.tsx

@@ -169,7 +169,10 @@ export const ChatMessagesContainer = ({
       <ConversationContent className="flex flex-1 flex-col gap-6 px-3 py-6">
         {headerSlot}
         {isLoading && messages.length === 0 && (
-          <div className="flex min-h-full flex-1 items-center justify-center">
+          <div
+            className="flex flex-1 items-center justify-center"
+            style={{ minHeight: "calc(100vh - 12rem)" }}
+          >
             <LoadingSpinner className="text-neutral-600" />
           </div>
         )}