Truecolor underline not working in tmux over ssh

### Checklist

- [x] Using latest version on TestFlight
- [x] Read the [docs](http://blink.sh/docs/)
- [x] Searched for [existing GitHub issues](https://github.com/blinksh/blink/issues)

### Configuration

Blink version: v18.1.1.977 (TestFlight)
iOS version: iPadOS 18.3.1

### Describe the bug

What's happening:
1. ssh to remote server and start tmux with the following `~/.tmux.conf`.
```
set-option -sa terminal-overrides ",xterm-256color:Tc"
set-option -s default-terminal "tmux-direct"
```
2. `cat colors.txt` [colors.txt](https://github.com/user-attachments/files/18924798/colors.txt)
3. Observe that in Ghostty "truecolor underline" has colored underline, while in Blink the underline is bland.

Expected behavior: truecolor underline works in every scenario as shown below.

Screenshots

Native Blink and over ssh:

Blink over ssh with tmux:

Ghostty on macOS over ssh with tmux:

Reproduce the report on iPadOS 18.3.1 with Blink v18.1.1.977: SSH to a remote server, start tmux using the shown ~/.tmux.conf, and run colors.txt. Compare the underline rendering with the supplied screenshots; done means the truecolor underline is colored in Blink over SSH through tmux.

Written by the indexing model from the issue text.