tui: assistant prose has uneven left and right margins

Summary

In the TUI, assistant prose sits about 5 columns from the left edge but only
about 2 from the right, so long responses look off-balance against the
scrollbar edge.

Environment

• opencode version: 2.0.3 (also present on the current v2 development branch)
• OS: Darwin 27.0.0 (darwin arm64)
• Terminal: ghostty (TERM=xterm-ghostty, COLORTERM=truecolor)
• Shell: /opt/homebrew/bin/fish
• Install/channel: latest
• Active plugins: None found in config

Reproduction

1. Open any session in the TUI.
2. View an assistant response with a few long paragraphs of prose.
3. Compare the empty space to the left of the text with the space to the right.

Expected Behavior

Regular prose has symmetric side margins.

Actual Behavior

The left margin is ~5 columns (2 from the message column container + 3 from
the text part) while the right margin is ~2 columns, so wrapped text runs much
closer to the right edge and scrollbar.

Additional Context

Consistently reproducible at any terminal width (on narrow terminals under 44
columns the same asymmetry shows as 4 vs 1). The text part in
packages/tui/src/routes/session/message-parts.tsx applies paddingLeft={3}
with no paddingRight. Suggested fix: mirror the left inset on the right.