fix(tui): keep home shortcuts right-aligned

[opencode-agent]

Summary

• preserve an empty footer spacer when the home prompt has no session directory
• keep the home shortcuts on the right after the idle-directory footer change

Checks

• bun typecheck (packages/tui)
• OpenCode Drive visual reproduction on origin/v2 + 34e5809059, then verification with this fallback restored

[github-actions]

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