`"theme": "system"` TUI black background flash on startup

Description

With "theme": "system" the TUI flashes a black (unthemed) background for ~1s on startup, then repaints with the correct theme. Reproduces in opencode2 --standalone, so it is not related to the background service. Appears to be a regression of the V1 issue #4472 (fixed by PR #5842) in the rewritten V2 TUI.

Plugins

n/a

OpenCode version

opencode2 v0.0.0-next-17444

Steps to reproduce

1. Set the TUI theme to system in ~/.config/opencode/cli.json:

   { "theme": { "name": "system" } }
   

2. Launch opencode2 --standalone.
3. Observe the screen: it shows a solid black (terminal-default) background for ~1 second, then repaints with the system theme colors.

Screenshot and/or share link

https://github.com/user-attachments/assets/454e676c-88c8-46bc-83b8-8a0f77b370ff

Operating System

Linux 7.1.4-arch1-1 (Arch), x86_64

Terminal

Alacritty (default black background, no explicit colors configured). TERM=xterm-256color, COLORTERM=truecolor. Reproduces both directly in Alacritty and inside tmux (TERM_PROGRAM=tmux)