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/index.html

@@ -649,6 +649,30 @@
                                         >
                                     </span>
                                 </button>
+                                <button
+                                    class="menu-list-row menu-list-button"
+                                    id="editor-poc-btn"
+                                    type="button"
+                                    role="menuitem"
+                                >
+                                    <span
+                                        class="menu-list-icon"
+                                        aria-hidden="true"
+                                    >
+                                        <i
+                                            data-lucide="pencil"
+                                            style="width: 17px; height: 17px"
+                                        ></i>
+                                    </span>
+                                    <span class="menu-list-content">
+                                        <span class="menu-list-title"
+                                            >Editor POC</span
+                                        >
+                                        <span class="menu-list-subtitle"
+                                            >Nouvel editeur (demo)</span
+                                        >
+                                    </span>
+                                </button>
                             </div>
                         </div>
                     </div>