Feature: Color-coded message bubbles by role in conversation view

Summary

In long sessions, it's hard to visually distinguish between user messages, assistant responses, and tool/activity output (Bash, Read, Edit, etc.). The conversation view currently renders everything in a similar monochrome style.

Proposal

Add color-coded visual differentiation for each message role — similar to how iMessage distinguishes sent vs. received messages:

• User messages — one color/style (e.g., blue bubble or left-aligned with distinct background)
• Assistant responses — a different color/style (e.g., gray or right-aligned)
• Tool activity (Bash, Read, Edit, Grep, etc.) — a third style (e.g., muted/dimmed, collapsible, or with a distinct accent color)

This would apply to both the VS Code extension and the terminal client (using ANSI colors for terminal).

Why

When scrolling back through a conversation to find something specific, every block looks the same. Color/style coding by role would make it immediately scannable — you could spot your own inputs, find where Claude responded, and skip past tool execution output at a glance.

Prior Art

• iMessage (blue/gray bubbles)
• Slack (distinct sender colors + bot styling)
• GitHub Copilot Chat (user vs. assistant styling)
• ChatGPT (alternating background shading)

Even a simple alternating background shade or left/right alignment difference would be a big UX win.