Menu bar: icon only, drop "Markdown" text

[jzone3]

Summary

Per user feedback (tweet from @quang__design), the menu bar item took too much horizontal space. The status item now shows only the icon: removed item.button?.title = "Markdown" and set imagePosition = .imageOnly in setUpStatusItem().

Link to Devin session: https://app.devin.ai/sessions/44b3a07e6e4448ce80bd62b863a672ed
Requested by: @jzone3

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.