Design: ask follow-up questions about selected answer text

[alectimison-maker]

Problem

When a WebBrain answer contains a detail that needs clarification, the user currently has to copy part of the answer manually and paste it into the composer. This breaks the reading-to-follow-up flow, especially for long streamed answers.

Proposed design

• Allow users to select text inside an assistant answer in the side panel.
• Show a compact, accessible Ask WebBrain action anchored near the selection.
• Clicking the action should place the selected text into the existing composer as a clearly delimited quote, then focus the composer without sending automatically.
• Leave the quoted text editable so the user can add a question, correct the excerpt, or cancel it before sending.
• Dismiss the action when the selection is cleared, the user clicks elsewhere, or the conversation/tab changes.
• Keep the interaction local to the side-panel UI and mirror it in the Chrome and Firefox builds; no new background protocol or persistence format should be needed for the first slice.

Acceptance criteria

• Selecting assistant answer text exposes the action without affecting normal text selection, copy buttons, or message metadata controls.
• The quote preserves the selected plain text and cannot inject markup into the side panel.
• The composer contains the quote and places the caret where a follow-up question can be entered.
• Clicking outside or clearing the selection removes the action without changing the composer draft.
• Keyboard and reduced-motion behavior remain usable.
• The behavior is covered by focused tests for quote construction and the selection-action lifecycle.

Alternatives considered

1. Add another context-menu entry: discoverability is lower and it takes the user away from the answer they are reading.
2. Send the selected text immediately: it removes the opportunity to phrase a precise follow-up and is surprising for an interaction that starts as text selection.

The initial implementation should keep the quote representation simple and compatible with the current textarea composer; richer quote chips or cross-turn answer anchors can be follow-up work.