fix(vscode): remove copy button and improve spacing for user messages

[Shreya657]

Description

Closes #1093

Fixes the cramped and visually awkward layout of user messages in the VS Code chat panel, especially for short prompts such as hi.

This implements the proposed changes from the issue:

• Removes the copy button from user messages while keeping it available for assistant messages.
• Increases the spacing between the user message bubble and its timestamp.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changeset

• Added a changeset (pnpm changeset) describing this change for the changelog

Docs-only or internal chores need no changeset (or run pnpm changeset --empty to note that intentionally).

This is a small VS Code UI improvement with no user-facing API or behavior changes.

Changes

• Removed the copy button from user messages since users generally don't need to copy their own prompts.
• Increased the spacing between the user message bubble and its timestamp from mt-1 to mt-2.
• Kept the copy button for assistant messages.

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Manual Testing

• Verified the VS Code chat panel UI locally.
• Verified that user messages no longer display the copy button.
• Verified that assistant messages still display the copy button.
• Verified the increased spacing below user messages, including short prompts.
• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Checklist

• If this was for an open issue, I was assigned to it
• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging (see CONTRIBUTING.md)