fzf border not visible on Linux TTY

### Checklist

- [x] I have read through the manual page (`man fzf`)
- [x] I have searched through the existing issues
- [x] For bug reports, I have checked if the bug is reproducible in the latest version of fzf

### Output of `fzf --version`

0.65.2

### OS

- [x] Linux
- [ ] macOS
- [ ] Windows
- [ ] Etc.

### Shell

- [x] bash
- [ ] zsh
- [ ] fish

### Problem / Steps to reproduce

When launching fzf on a Linux TTY with `TERM=linux`, fzf seems not capable of drawing a border around the list of files.

I can reproduce this by:
- booting into a Linux system
- jump to a TTY or not boot into a desktop manager
- run `fzf --margin=1 --border=rounded`
- see missing border (sometimes "replaced" by the characters which were drawn at the same positions before)

Screenshot of a qemu VM (but I can also reproduce this on actual hardware):

Reproduce the issue by running `fzf --margin=1 --border=rounded` on a Linux TTY with `TERM=linux`, then compare the rendering with a supported terminal environment. Trace the border-rendering path from the fzf command entry point and verify that the rounded border is visible without leaving stale characters in the TTY output.

Written by the indexing model from the issue text.