Reworked Stack Auth demo (stack-auth#624)

![image](https://github.com/user-attachments/assets/453c4ea5-9649-4a34-94b1-15f6c8bcecde)

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> [!IMPORTANT]
> Reworked Stack Auth demo with UI enhancements and code refactoring for
improved user experience and maintainability.
> 
>   - **UI Enhancements**:
> - Updated `global.css` to change background colors for light and dark
themes.
> - Added `Image` components in `page-client.tsx` and `header.tsx` for
visual elements.
> - Refactored `page-client.tsx` to use `Card`, `CardContent`,
`CardFooter`, and `CardHeader` for user information display.
>   - **Code Refactoring**:
> - Removed `stack-scope` class from `SelectTrigger` in `select.tsx`.
> - Updated button layout and text in `page-client.tsx` for
sign-in/sign-up and user information.
> - Enhanced header layout in `header.tsx` with logo and theme toggle
improvements.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=stack-auth%2Fstack-auth&utm_source=github&utm_medium=referral)<sup>
for c7a9f30. It will automatically
update as commits are pushed.</sup>


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Author: TheCactusBlue

examples/demo/public/images/wave.png

@@ -4,12 +4,12 @@
 
 
 :root {
-  --background: white;
+  --background: rgb(240, 240, 240);
   --foreground: black;
 }
 
 html:has(head > [data-stack-theme="dark"]) {
-  --background: black;
+  --background: rgb(16, 16, 18);
   --foreground: white;
 }