feat(ui): remove vertical borders from input prompt for easier copy/paste

[afarber]

Summary

Remove vertical border characters from the input prompt box when not in alternate buffer mode, making copy/paste easier in the default terminal mode.

Details

The input prompt previously used a full rounded box (╭──╮ │ │ ╰──╯) in all modes. The vertical bars would get included when selecting text, messing up copy/pastes.

Now in non-alternate buffer mode (the default), only horizontal lines are shown at top and bottom. The full rounded box is preserved in alternate buffer mode where copy/paste already needs special handling.

Related Issues

Fixes #14385 ( sub task of #14382 )

How to Validate

1. Run npm run build && npm start
2. Verify the input prompt shows only horizontal lines (no vertical borders)
3. Try selecting and copying text from the prompt area - should be clean
4. Edit ~/.gemini/settings.json and add "ui": { "useAlternateBuffer": true }
5. Restart and verify the full rounded box with vertical borders appears

Pre-Merge Checklist

• [ ] Updated relevant documentation and README (if needed)
• [x] Added/updated tests (if needed)
• [ ] Noted breaking changes (if any)
• [x] Validated on required platforms/methods:
  • [x] MacOS
    • [x] npm run
    • [ ] npx
    • [ ] Docker
    • [ ] Podman
    • [ ] Seatbelt
  • [ ] Windows
    • [ ] npm run
    • [ ] npx
    • [ ] Docker
  • [x] Linux
    • [x] npm run
    • [ ] npx
    • [ ] Docker