dotnetup: Add scroll indicator hint to channel selector

[nagilson]

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).