fix(assistant): make blockquote and hr tags look not broken COMPASS-9872

[lerouxb]

This PR:

• consolidates all the bits of css overrides that all only apply to the chat into one so we have one common class, one for light, one for dark
• does the bare minimum to make blockquote tags not look completely ridiculous. I just guessed at colours and the line thickness - we can easily change it once we have designs
• does the bare minimum to make hr tags that are alone in a message show up and not be 1px in length.

light:

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dark:

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[Copilot]

Pull Request Overview

This PR consolidates CSS overrides for the Assistant chat component and fixes the visual appearance of blockquote and hr HTML elements. The changes improve the rendering of these elements by providing proper styling in both light and dark themes.

• Consolidates separate CSS style objects into unified light/dark theme styles
• Adds proper styling for blockquote elements with border and padding
• Fixes hr tags to have minimum width when displayed alone in chat messages

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