Fix markdown editor preview mode and dirty close behavior

[wgqqqqq]

Summary

• add a Preview / Markdown mode toggle for the Markdown editor and keep dirty tracking consistent across mode switches
• allow deleting an empty markdown table with backspace in the visual editor
• prevent programmatic markdown content sync from entering the undo history so Cmd+Z does not mark untouched documents dirty
• route dirty tab close confirmation through the shared confirm dialog and avoid duplicate close handling in canvas tab flows

Verification

• pnpm run type-check:web
• pnpm run lint:web