[FEATURE] Support Shift+PageUp/PageDown to extend text selection across pages in Scroll mode

Description

In fullscreen/Scroll mode, I can select text line-by-line using Shift+Arrow keys. However, there is no way to extend the selection across pages using keyboard only.

Expected behavior: Hold Shift and press fn+Up/fn+Down (PageUp/PageDown on Mac) to extend the text selection by a full page, similar to how most text editors and terminals handle Shift+PageUp/PageDown.

Actual behavior: Shift+fn+Up/Down does not extend the selection. Only line-by-line selection (Shift+Up/Down) works, which is impractical for selecting large blocks of text spanning multiple pages.

Use Case

When reviewing long Claude responses, I often need to select and copy a section that spans several pages. Without mouse scroll, the only option is Shift+Arrow key pressed dozens of times, which is not practical.

Proposed Solution

Add selection:extendPageUp and selection:extendPageDown actions in the Scroll context, mapped to Shift+PageUp / Shift+PageDown by default. This would allow keyboard-only users to efficiently select text across pages.

Environment

• Claude Code version: 2.1.92
• OS: macOS 26.3.1 (arm64)
• Machine: MacBook Pro, Apple M4 Pro, 48 GB
• Terminal: iTerm2