Cmd+Down should page down (macOS/VSCode consistency)

> I always press Cmd down arrow to page down, but the shortcut that actually works is option down arrow. Not sure if I should just learn the Pad way or if we should make Pad shortcuts consistent with VSCode.

## Claude Opus 4.6's Interpretation

Page down is bound to `alt+down` (`src/pad/app.py:1075`). In VSCode and most macOS apps, Cmd+Down moves to end of document. The user is genuinely unsure whether Pad should adopt VSCode conventions or whether this is just a learning curve thing.

### Possible solutions

- Add Cmd+Down/Cmd+Up as aliases for page down/up
- Match VSCode conventions: Cmd+Down = end of file, Option+Down = page down (current)
- Keep current bindings and document the keybindings more visibly so users can discover them
- Some combination: adopt the most common macOS/VSCode shortcuts but keep Pad-specific ones where they don't conflict