fix: make one-line extension widgets expandable

[Shujakuinkuraudo]

Summary

• Treat every non-empty extension widget as expandable in the compact status bar.
• Keep one-line widgets collapsed by default while preserving the existing empty-widget behavior.
• Add regression coverage for one-line and empty widgets.

Testing

• npm test (588 tests passed)
• node_modules/.bin/tsc --noEmit
• npm run lint
• npm run build

AI assistance

OpenAI Codex was used to investigate the widget behavior, implement the fix, and run the tests. The resulting diff was independently reviewed before submission.

Closes #538