[FEATURE]: Quote selected assistant text into the composer

Feature has been suggested before

Related request #17493 was automatically closed after 60 days of inactivity, with an invitation to open a new issue if the feature remained relevant.

Related TUI work: #44443.

Problem

When reading a long assistant response, I often want to ask a follow-up about one specific passage.

The current Desktop/Web workflow is:

1. Select text
2. Copy it
3. Click the composer
4. Paste it
5. Type the follow-up

Manually returning focus to the composer makes this interaction unnecessarily slow.

Ctrl+L can focus the input, but it is still an additional action and is not obvious during a mouse-based selection workflow.

Preferred solution

When text is selected inside an assistant response, show a small Quote action near the selection.

Activating it should:

• Add the selected plain text to the composer as quoted context
• Preserve any existing draft
• Immediately focus the composer
• Place the caret after the inserted context, ready for the follow-up
• Avoid automatically submitting anything

The intended workflow is Select → Quote → Type immediately, without another click on the composer.

Initial scope could support only plain-text selection within a single assistant message.

Smaller fallback

If the dedicated Quote action is considered too large in scope, automatically focus the composer after the user copies selected assistant-response text.

That would reduce the workflow to:

1. Select text
2. Copy
3. Paste immediately
4. Type the follow-up

This fallback would not automatically paste, quote, or submit anything. It would only activate the composer, preserve the existing draft, and place the caret at its end.

It should apply only when copied text originated from an assistant response, not when copying unrelated interface content.

Design mockups

Mockups illustrating the Quote action and the focused composer state will be attached after publication. They are design proposals, not screenshots of existing functionality.

Why this helps

This makes targeted follow-ups faster while reading long responses, especially in the Desktop app where text selection is already a natural mouse interaction.

It also aligns Desktop/Web with the intent of the selection-to-prompt workflow currently proposed for the TUI in #44443.