Add keyboard shortcuts for scrolling

[wayyoungboy]

Summary

• add MOD+PageUp and MOD+PageDown shortcuts to inject vertical scroll events
• document the new shortcuts in CLI help and shortcuts.md
• add a focused unit test for the keyboard shortcut to scroll mapping

Fixes #6891

Validation

• RED: cc -Iapp/src app/tests/test_keyboard_shortcuts.c -o build-test-keyboard-shortcuts failed with fatal error: 'keyboard_shortcuts.h' file not found
• GREEN: focused compile/run with a temporary SDL keycode stub passed:
  cc -I"$tmpdir" -Iapp/src app/tests/test_keyboard_shortcuts.c app/src/keyboard_shortcuts.c -o "$tmpdir/test_keyboard_shortcuts" && "$tmpdir/test_keyboard_shortcuts"
• git diff --check

Full Meson build was not run locally because this environment lacks meson and the required SDL3/FFmpeg pkg-config dependencies.