fix(dom): avoid leaking cell background to container

Summary

• stop mirroring the bottom-right cell background onto the terminal container
• keep per-cell and full-row backgrounds rendered normally
• add a regression test for tmux/status-line-style bottom-right backgrounds

Why

Full-width UI elements such as tmux status bars can color the bottom-right cell. Mirroring that cell onto the container makes the status bar background leak into the entire terminal.

Test

• corepack pnpm --filter @wterm/dom test
• corepack pnpm --filter @wterm/dom type-check