[DASH-444] Nebula - Update login flow (#5750)

## Problem solved

Short description of the bug fixed or feature added

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---

## PR-Codex overview
This PR focuses on refactoring and updating the chat and login components in the `nebula-app`, enhancing the handling of chat sessions, prompts, and user interactions.

### Detailed summary
- Added `initialPrompt` to `ChatPageContent` and `EmptyStateChatPageContent`.
- Simplified `EmptyStateChatPageContent` props.
- Introduced `LoginAndOnboardingPageContent` for better login handling.
- Updated `NebulaLogin` to use `NebulaLoginPage`.
- Refactored message handling and session management in `ChatPageContent`.
- Removed unnecessary props in `Chatbar` and `EmptyStateChatPageContent`.

> ✨ Ask PR-Codex anything about this PR by commenting with `/codex {your question}`

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apps/dashboard/src/app/login/LoginPage.tsx

@@ -77,6 +77,20 @@ export function LoginAndOnboardingPage(props: {
         </header>
       </div>
 
+      <LoginAndOnboardingPageContent
+        account={props.account}
+        redirectPath={props.redirectPath}
+      />
+    </div>
+  );
+}
+
+export function LoginAndOnboardingPageContent(props: {
+  account: Account | undefined;
+  redirectPath: string;
+}) {
+  return (
+    <div className="relative flex grow flex-col">
       <main className="container z-10 flex grow flex-col justify-center gap-6 py-12">
         <ClientOnly
           ssr={

apps/dashboard/src/app/nebula-app/(app)/chat/[session_id]/page.tsx

@@ -44,6 +44,7 @@ export default async function Page(props: {
       session={session}
       type="new-chat"
       account={account}
+      initialPrompt={undefined}
     />
   );
 }

apps/dashboard/src/app/nebula-app/(app)/chat/page.tsx

@@ -27,6 +27,7 @@ export default async function Page() {
       session={undefined}
       type="new-chat"
       account={account}
+      initialPrompt={undefined}
     />
   );
 }

apps/dashboard/src/app/nebula-app/(app)/components/ChatBar.tsx

@@ -5,11 +5,8 @@ import { AutoResizeTextarea } from "@/components/ui/textarea";
 import { cn } from "@/lib/utils";
 import { ArrowUpIcon, CircleStopIcon } from "lucide-react";
 import { useState } from "react";
-import type { ExecuteConfig } from "../api/types";
 
 export function Chatbar(props: {
-  updateConfig: (config: ExecuteConfig) => void;
-  config: ExecuteConfig;
   sendMessage: (message: string) => void;
   isChatStreaming: boolean;
   abortChatStream: () => void;