[Bug] selection:copy keybinding fails to copy keyboard-extended text selections in fullscreen mode

Bug Description
Title: selection:copy (Ctrl+Shift+C) and Cmd+C don't copy a keyboard-extended text selection; only plain Ctrl+C does

Summary:
In fullscreen rendering mode, after creating a text selection with the mouse and extending it via shift+arrows (selection:extendLeft/Right/Up/Down), the configured selection:copy action (default Ctrl+Shift+C) and Cmd+C fail to copy the extended selection. Ctrl+Shift+C produces an error beep and leaves the selection intact but copies nothing. Only plain Ctrl+C correctly copies the extended selection.

Steps to reproduce:

1. Enable fullscreen rendering ("tui": "fullscreen").
2. Mouse-drag to select text in the conversation (Claude Code reports "N chars copied").
3. Press shift+arrows to extend the selection (highlight grows as expected).
4. Press Ctrl+Shift+C (the bound selection:copy) → beeps, copies nothing.
5. Press Cmd+C → copies only the original mouse selection, not the extension.
6. Press plain Ctrl+C → correctly copies the full extended selection (confirmed by Claude Code's message and clipboard contents).

Environment:

• macOS
• Reproduced in both Terminal.app and Ghostty → terminal-independent, points to Claude Code, not the emulator.

Expected: The configured selection:copy keybinding (Ctrl+Shift+C) should copy whatever the active selection currently is, including keyboard-extended selections — matching plain Ctrl+C.

Impact: The documented/configurable copy action is inconsistent with the working (undocumented) plain-Ctrl+C path, which is confusing and makes the selection:copy keybinding effectively broken for keyboard-extended selections.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.177
• Feedback ID: 4e4753bc-a2d6-4147-94db-58abbd8b035b

Errors

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