Fix: Dark Mode Background Contrast in Auth Page

## Problem
In Dark Mode, the background of the left-side section (Authentication.tsx) is too light (bg-muted). This causes the "podium" illustration to merge with the background, losing visual definition.

## Proposed Change
Update the background utility class to explicitly use a darker shade (gray-950) when in dark mode, while keeping the original color for light mode.

## Before

## After

File: src/components/Authentication.tsx

Change:

```bash

// LeftSection component div