Slash command dropdown overflows on tall windows; input box reposition shifts vertically

[jorgeraad]

Two related issues with the prompt input UI:

1. Dropdown overflows on tall windows. With ~50 skills installed, the slash command dropdown renders far more items than fit in the visible area when the terminal is tall — items get clipped off the top. The sizing calculation should cap visible items so they actually fit.

2. Input box drifts vertically. Recently the text input started centering itself vertically, so adding a newline or changing the dropdown item count pushes the top edge up and the bottom edge down. Previously the input was anchored to a fixed position so it didn't jump around as content above/below it changed. Would like to revert to the anchored behavior.

[jorgeraad]

@cursor please take a look at this

[cursor]

Both fixes are complete. Here's a summary of what I implemented:

Changes in src/tui/components/chat/home-view.tsx:

• Dropdown overflow fix: The maxVisibleSuggestions calculation now divides available vertical space by 2 to account for each suggestion item potentially occupying 2 terminal rows (label + wrapped multi-line description). This prevents the dropdown from exceeding the viewport on tall windows with many skills.

• Input anchoring fix: Replaced justifyContent="center" + height="100%" on the content column with a computed marginTop (contentTopMargin). This positions the content block at the vertical center based on the base content height (excluding the dropdown), so the input stays at a fixed position when the dropdown appears or the textarea grows. The dropdown expands downward from the anchored position instead of shifting the whole block.

• Cleanup: Removed the unused showHelpText variable that was vestigial from a removed feature.

All CI checks pass (TypeScript, lint, tests), and I verified both fixes manually in the TUI.