Replace the four lines of `==` rules + bilingual title with a centred
rounded box so the eye lands on a single visual element instead of two.
The new tui_render_welcome():
- centres horizontally and vertically (about a third of the way down)
- shows TNT + version, then 中文 subtitle, then English caption with
decreasing visual weight (bold cyan / default / dim grey)
- falls back to a plain "TNT $version — anonymous chat over SSH" line
when the terminal is too narrow for the frame
- declared in tui.h, called from input.c read_username()
read_username()'s prompt becomes a single short bilingual hint
("请输入用户名 (留空 anonymous): ") that lives below the box.
- Implement SSH server using libssh for secure connections
- Replace insecure telnet with encrypted SSH protocol
- Add automatic terminal size detection via PTY requests
- Support dynamic window resize (SIGWINCH handling)
- Fix UI display bug by using SSH channel instead of fd
- Update tui_clear_screen to work with SSH connections
- Add RSA host key auto-generation on first run
- Update README with SSH instructions and security notes
- Add libssh dependency to Makefile with auto-detection
- Remove all telnet-related code
Security improvements:
- All traffic now encrypted
- Host key authentication
- No more plaintext transmission
