Selection context menu lacks a 'copy selection' option; existing copy actions ignore selection scope

Description

When highlighting a small piece of text within an assistant message, a context menu appears with these options:

• Copy message
• Copy as Markdown
• Attach selection as context
• Pin as chapter

The menu is triggered by making a text selection, which implies the actions should operate on that selection. However, Copy message and Copy as Markdown both ignore the highlighted range and copy the entire message instead.

This is likely intended behavior for those two actions (grab the full message in different formats), and Ctrl-C does correctly copy just the highlighted selection as plain text. So the underlying issue isn't that copying is broken — it's that:

1. The menu only appears when text is selected, which sets an expectation that its actions apply to that selection.
2. There's no explicit "Copy selection" item in the menu itself, even though the menu is selection-triggered.

Suggested fix

Add a "Copy selection" (plain text) option to this context menu, distinct from "Copy message" / "Copy as Markdown," so the menu's available actions match user expectations set by it appearing only on a text selection.

Repro steps

1. In a chat with an assistant response, highlight/select a small word or phrase within the message (not the whole message).
2. Right-click (or however the menu is triggered) to bring up the selection context menu.
3. Click "Copy message" or "Copy as Markdown."
4. Paste — the entire message is copied, not just the highlighted selection.

Screenshot

A screenshot was provided in the original report showing the context menu with "ESP32-8048S043C-I" highlighted within a longer message; menu options shown: Copy message / Copy as Markdown / Attach selection as context / Pin as chapter.

<img width="809" height="250" alt="Image" src="https://github.com/user-attachments/assets/26fd9ca9-31ca-4c71-af07-e86a0405f2c7" />