webui: add responsive chat width option to webui (#18067)

[ImadSaddik]

Description

This PR addresses issue #18067 by introducing a "Responsive chat width" option in the Web UI settings.

Currently, the chat interface has a fixed maximum width (48rem), which results in significant unused screen space on large monitors (2K, 4K, ultrawide). This change allows users to opt in to a wider layout that scales better with higher resolutions.

Changes

1. Added responsiveChatWidth to the settings dialog.
   • Default: false (maintains existing behavior).
   • Enabled: Applies new responsive width classes based on screen size.
2. Added 3xl (1920px), 4xl (2400px), and 5xl (2880px) breakpoints in app.css to target high-resolution displays specifically.
3. Updated the following components to respect the new setting:
   • ChatScreen
   • ChatMessages
   • ChatMessageAssistant
   • ChatForm
   • ChatProcessingInfo
4. The specific width values in MAX_WIDTH_CLASSES are a starting point. I am happy to adjust them if the layout feels too wide or too narrow on specific screens.

Screenshots

Captured on a 2K display.

Before	After

Captured on a 1K display.

Before	After

How to Test

1. Go to Settings > General.
2. Toggle Responsive chat width to "On".
3. Resize the window or view on a large screen (1080p+) to see the layout expand.

Motivation

On 2K and 4K screens, the previous fixed width used only a small percentage of the available viewport (~30% on 2K and ~20% on 4K)