[FEATURE] Desktop: keyboard shortcut for "Reply" on selected transcript text (e.g. Cmd+E / Cmd+Shift+R)

## Feature request

**Area:** desktop app (Code tab), macOS.

### Problem

When text is selected in the transcript, a "Reply" action appears in a floating popup. It is the fastest way to quote a specific fragment of Claude's answer and ask a follow-up about it, but it is mouse-only: after selecting text with the keyboard (or with the mouse) you still have to move the pointer to the popup and click the button.

The keyboard shortcuts page for the desktop app (https://code.claude.com/docs/en/desktop) does not list any shortcut for this action, and it cannot be bound via `~/.claude/keybindings.json` because that file only covers the terminal CLI.

### Proposal

Add a keyboard shortcut that triggers "Reply" for the current selection, e.g.:

- `Cmd+E` (free according to the documented shortcut list), or
- `Cmd+Shift+R` (`Cmd+Shift+E` is already taken by the effort menu).

Behaviour: with a non-empty selection in the transcript, the shortcut does exactly what clicking "Reply" does (quotes the selection into the composer and focuses it). With no selection it is a no-op.

Ideally the shortcut would also show up in the `Cmd+/` shortcuts overlay.

### Why

Quote-and-ask is a very frequent interaction while reviewing long answers, diffs and logs. Making it keyboard-driven removes a mouse round trip on every follow-up. This is also in the spirit of the keyboard-operability tracking issue #90761.

### Environment

- Claude desktop app, Code tab
- macOS (Darwin 25.6, MacBook)

Start with the desktop app keyboard shortcuts page at https://code.claude.com/docs/en/desktop and inspect the existing Cmd+/ shortcuts overlay and transcript selection Reply action. Check how desktop shortcuts are registered, then verify that the chosen shortcut quotes a non-empty selection, focuses the composer, is a no-op without selection, and appears in the overlay.

Written by the indexing model from the issue text.