feat(editor): migrate markdown editor to Tiptap and fix macOS menu routing

[wgqqqqq]

Summary

• route macOS Monaco edit commands explicitly and unify editor menu routing on macOS
• replace the legacy IR markdown editor with a Tiptap-based editor and inline AI preview flow
• fix markdown image rerender scroll jumps and add the supporting editor services, extensions, and tests

Testing

• Not run (not requested)