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

frontend/app.js

@@ -4069,6 +4069,15 @@
     });
   }
 
+  function initEditorPocBtn() {
+    var btn = document.getElementById("editor-poc-btn");
+    if (!btn) return;
+    btn.addEventListener("click", function() {
+      closeHeaderMenu();
+      window.open("/editor-poc", "_blank");
+    });
+  }
+
   function initHelpNavigation() {
     const helpContent = document.querySelector(".help-content");
     const helpBody = document.getElementById("help-body");
@@ -7819,6 +7828,7 @@
     initSidebarTabs();
     initHelpModal();
     initConfigModal();
+    initEditorPocBtn();
     initSidebarFilter();
     initSidebarResize();
     initEditor();