fix(app): resize home session tab indicator

[opencode-agent]

Summary

• resize the Home session open-tab indicator to 12px tall and 2px wide
• position the indicator 4px from the session avatar

Validation

• bun typecheck in packages/app
• bunx prettier --check packages/app/src/pages/home.tsx
• focused Chromium geometry check: 2px × 12px with a 4px avatar gap

[github-actions]

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