bug : Forwarded Message UI to match Rocket.Chat native UI

Description

Currently, when a message is quoted or forwarded, EmbeddedChat renders it using a UI that differs significantly from the native Rocket.Chat interface.

The main discrepancies in TextAttachment.js are:

1. The @ symbol: EmbeddedChat mistakenly prefixes the quoted user's name with an @ symbol (e.g., @username). Rocket.Chat does not use the @ symbol for quoted authors.
2. Double Box Layout: The quoted text is currently rendered inside a nested Box that applies a grey background and a full border, making it look like a box inside a box. Rocket.Chat simply renders the quoted text directly inside the outer attachment wrapper without applying an inner background or border.
3. Typography: The author's name in the quote is not styled with the correct font weight to match the native client.

EmbeddedChat :

RocketChat :

Expected Behavior

Quoted messages should look exactly like they do in Rocket.Chat:

• The username should not have an @ prefix and should have a medium font weight (500).
• The quoted message text should not be wrapped in an extra background/border box.

Steps to Reproduce

1. Forward or quote a message in the chat.
2. Observe the message in EmbeddedChat.
3. Notice the @ symbol and the double-box layout applied to the quoted markdown text.

Start in TextAttachment.js, where forwarded and quoted messages are rendered. Compare the current author label, nested Box, and typography with the expected Rocket.Chat appearance shown in the issue. Done means the username has no @ prefix and uses weight 500, while the quoted text no longer has the extra background or border.