Cannot select a substring of a chat message (from user) to copy

## Description

In the Pi chat panel, users can click the copy button on a message to copy the entire message, but cannot click and drag to select any part of the message text (user bubbles) to copy a snippet.

Because `body` has `user-select: none` in `apps/web/src/index.css` and content is expected to opt back in (`.tr-prose-chat`, `data-selectable`, etc.), chat message containers (`USER_BUBBLE` / `USER_BUBBLE_BASE` / `PlainUserTurn` and assistant message wrappers in `apps/web/src/chat/turns.tsx`) currently do not include `.tr-prose-chat` or `data-selectable` (or `user-select: text`). As a result, message text inherits `user-select: none` and cannot be selected.

## Steps to reproduce

1. Open a workspace and open or start a chat session.
2. Send a message or wait for an assistant response.
3. Try clicking and dragging the cursor over words or lines within the message bubble to highlight/select a portion of the text.

## Expected behavior

Users can click and drag to highlight/select any part of a user or assistant chat message and copy the selected text, while preserving whole-message copy via the copy button.

## Actual behavior

No text selection occurs when dragging over the message text; the cursor cannot select substrings within the message.

## Environment

- OS: macOS 26.6.2 (Darwin arm64)
- Bun version (`bun --version`): 1.4.2
- Node version (`node --version`): v26.8.2
- ThinkRail version / commit: 6a836d3fe (upstream/main)

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🤖 Generated by AI agent

Start with apps/web/src/index.css to confirm the global user-select rule, then inspect the USER_BUBBLE, USER_BUBBLE_BASE, PlainUserTurn, and assistant message wrappers in apps/web/src/chat/turns.tsx. Verify the relevant containers opt into text selection without disrupting the existing copy-button behavior. Done means users can select and copy substrings from both user and assistant messages.

Written by the indexing model from the issue text.