Extend Workbench selected state across sidebar row

[cinderzhan]

Summary

• Extend the Workbench homepage selected background across the full sidebar row, including behind the mode switch.
• Keep the homepage and mode switch as separate buttons with their existing widths and click targets.
• Cover the selected and unselected container states in the sidebar tests.

Verification

• npx vitest run test/workbench-client.test.mjs — 136 passed
• git diff --check — passed

The commit is based on dataelement/dsh-desktop:main and contains only this sidebar selection change.