dotnetup: Add scroll indicator hint to channel selector

## Summary

The channel selector UI shows a maximum of ~5 items at a time but gives no indication that more items are available via scrolling. Users may not realize they can scroll.

## Current Behavior

The selector renders a list of channels with `▲`/`▼` arrows at the edges when items overflow, but there is no text explaining how to navigate.

## Proposed Behavior

Modify the hint line below the selector:

```
...
(use ↑↓ arrows to select and scroll)
```

Alternatively, integrate the hint into the selector header or footer. The hint should:
- Use `↑↓` arrow symbols (not ASCII `^`/`v`).
- Be dimmed/subtle so it doesn't distract from the options.
- Only appear when there are more items than fit on screen.

## Acceptance Criteria

- [ ] Scroll hint is visible when the channel list overflows.
- [ ] Hint is not shown if all items fit without scrolling.
- [ ] Hint uses arrow symbols (`↑↓`) and is styled dimly.
- [ ] Hint also applies to any other scrollable selectors in the walkthrough (e.g., mode selector if it overflows).