feat: Editor POC — multi-zone toolbar mockup (TOOLBAR_DESIGN_ANALYSIS.md)

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

backend/main.py

@@ -4039,6 +4039,14 @@ if FRONTEND_DIR.exists():
             return HTMLResponse(content=popout_file.read_text(encoding="utf-8"))
         raise HTTPException(status_code=404, detail="Popout template not found")
 
+    @app.get("/editor-poc")
+    async def serve_editor_poc():
+        """Serve the standalone Editor POC page (multi-zone toolbar demo)."""
+        poc_file = FRONTEND_DIR / "editor-poc.html"
+        if poc_file.exists():
+            return HTMLResponse(content=poc_file.read_text(encoding="utf-8"))
+        raise HTTPException(status_code=404, detail="Editor POC not found")
+
     @app.get("/{full_path:path}")
     async def serve_spa(full_path: str):
         """Serve the SPA index.html for all non-API routes."""