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/js/app.js

@@ -38,6 +38,7 @@ async function init() {
   Legacy.initSidebarTabs();
   Legacy.initHelpModal();
   Legacy.initConfigModal();
+  Legacy.initEditorPocBtn();
   Sidebar.initSidebarFilter();
   UI.initSidebarResize();
   Utils.initEditor();

frontend/js/config.js

@@ -218,6 +218,14 @@ function initHelpModal() {
   });
 }
 
+function initEditorPocBtn() {
+  var btn = document.getElementById("editor-poc-btn");
+  if (!btn) return;
+  btn.addEventListener("click", function() {
+    window.open("/editor-poc", "_blank");
+  });
+}
+
 function initHelpNavigation() {
   const helpBody = document.getElementById("help-body");
   const navList = document.getElementById("help-nav-list");
@@ -1289,6 +1297,7 @@ export {
   initHelpModal,
   closeHelpModal,
   initRecentTab,
+  initEditorPocBtn,
   loadAbout as initAboutSection,
   loadHiddenFilesSettings as initHiddenFilesConfig,
   switchSidebarTab,

frontend/js/legacy.js

@@ -25,7 +25,7 @@ import {
 } from './dashboard.js';
 
 // --- Re-exports from modules that already have these functions ---
-export { initSidebarTabs, initConfigModal, initHelpModal, initRecentTab } from './config.js';
+export { initSidebarTabs, initConfigModal, initHelpModal, initRecentTab, initEditorPocBtn } from './config.js';
 export { initSyncStatus } from './sync.js';
 export { DashboardRecentWidget } from './dashboard.js';