fix(tui): fill image message background

[opencode-agent]

Summary

• apply the user-message surface background to the outer transcript message container
• extend the same background behind image previews without changing tool-result image styling or message interactions

Validation

• bun typecheck (packages/tui)
• bun test (packages/tui): 678 passed, 5 skipped
• real TUI before/after at 100×32 with the same standalone session and image attachment

Requested by: @simonklee (Simon via Slack)