feat(web): add copy action for assistant messages

[byrafael]

What Changed

This adds a copy action for assistant messages in the chat timeline when the assistant text is non-empty.

The implementation reuses the existing message copy button and clipboard behavior already used elsewhere in the UI, and places the action next to the assistant message timestamp/duration metadata. Empty assistant responses still render as (empty response) but do not expose a copy action.

Tests were added to cover:

• copy action shown for assistant messages with text
• copy action shown for streaming assistant messages with buffered text
• no copy action for empty non-streaming assistant messages

Why

Assistant responses were not directly copyable from the timeline, which made it harder to reuse agent output. This change keeps the scope tight by reusing the existing copy control rather than introducing a new interaction pattern.

Keeping empty responses uncopiable avoids copying placeholder UI text instead of real assistant content.

UI Changes

Before:

• assistant responses could not be copied, only user content would show a copy button.

After:

• assistant responses show a copy button beside the timestamp/duration metadata when text is present
• empty assistant responses still show (empty response) with no copy button

^ On hover.

Video

Screen.Recording.2026-03-25.at.18.17.05.mov

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Low Risk
Low risk UI-only change that conditionally exposes an existing clipboard action on assistant timeline entries; main risk is minor layout/hover behavior regressions around message metadata.

Overview
Adds a hover/focus-visible MessageCopyButton to assistant messages in MessagesTimeline, positioned next to the timestamp/duration metadata.

The copy action only renders when the assistant message text is non-empty (including buffered text while streaming), avoiding copying the (empty response) placeholder.

Extends MessagesTimeline.test.tsx with coverage for copy button visibility for non-empty, empty, and streaming assistant messages.

^{Written by Cursor Bugbot for commit 77eb6c0. This will update automatically on new commits. Configure here.}

Note

Add copy button to assistant messages in chat timeline

Adds a MessageCopyButton to assistant messages in MessagesTimeline.tsx that appears on hover or focus near the timestamp. The button is only rendered when the message has non-empty text, hiding it for empty responses and active streaming with no buffered content.

^{Macroscope summarized 77eb6c0.}

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[byrafael]

My apologies. The commit messages were generated wrong 😭