Add standalone /editor-poc page demonstrating all 5 toolbar zones: - Zone 1: Top Fixed Bar (hamburger, title, AI glow button, save indicator) - Zone 2a: Slash Command Menu (/ on empty line, 15 commands, filtering) - Zone 2b: Bubble Menu (on text selection, formatting + AI dropdown) - Zone 3: Floating Quick Insert (per-line, Image/PDF/Code/Table/AI) - Zone 4: AI Side Panel (Ctrl+J toggle, chat, suggestions) - Zone 5: Drag & Drop overlay with dashed border Files: - new: frontend/editor-poc.html (46KB standalone vanilla JS) - backend/main.py: +/editor-poc route before catch-all - frontend/index.html: Editor POC button in header menu - frontend/js/config.js: initEditorPocBtn() function + export - frontend/js/legacy.js: re-export initEditorPocBtn - frontend/js/app.js + frontend/app.js: init call in both versions