feat: add terminal scrolling by lines and pages - #409

Description

Add keyboard shortcuts to scroll the terminal scrollback by lines and by pages:

• Ctrl+Shift+Up / Ctrl+Shift+Down — scroll up/down one line
• Shift+PageUp / Shift+PageDown — scroll up/down one page

The shortcuts are registered and refreshable in the terminal context and exposed in the Settings → Shortcuts tab. Page scrolling keeps one line of context to avoid losing orientation. The shortcuts deliberately do not occupy plain PageUp/PageDown or Shift+Up/Shift+Down, which remain available for shell/TUI input and history navigation.

In VI mode the new actions move the VI cursor, matching the existing mouse-wheel semantics; in alt-screen there is no scrollback, so the keys are propagated to the running program (vim, less, etc.) instead of being consumed.

How to Test

1. Open a terminal tab and run a command that produces enough output to exceed the screen height.
2. Press Ctrl+Shift+Up / Ctrl+Shift+Down and confirm the scrollback scrolls by one line each time.
3. Press Shift+PageUp / Shift+PageDown and confirm scrolling by one page while keeping one line of context.
4. Run a full-screen TUI program (e.g. vim, less, htop) and confirm the new shortcuts do not interfere with it (keys are passed through).

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)