Consolidate header export actions into a format menu #53

Stakeholder feedback: the editor header currently shows multiple adjacent export actions (Export, Export .md, Export .txt), which reads as duplicated and visually noisy.\n\nAcceptance criteria:\n- Header shows one clear Export entry point for file export.\n- Export formats such as Markdown and plain text are presented as choices inside that entry point instead of separate top-level buttons.\n- Import remains separate and clear.\n- Layout works in the existing dark/light theme chrome without clipping or duplicate-looking labels.\n- Browser/UI coverage verifies the export menu and available format choices.