VS Code: filter buttons and left/right justify for user vs AI messages

Feature request

Two specific features for the VS Code chat panel:

1. Left/right justification — user messages left-aligned, AI messages right-aligned (like iMessage/WhatsApp), with different background colors. This is a stronger visual signal than a subtle highlight — you can scan a long conversation instantly.

1. Filter buttons — two toggles at the top of the chat panel: "User" and "AI". Click one to hide the other side. When you need to re-read what you asked across a long session, hide the AI output and scroll just your messages.

Related: #34269, #29974 (general visual distinction requests — this issue is specifically about justify layout and filtering, not color/highlight tweaks).

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