feat: notes encore plus larges — max-w-6xl body + font 18px

Full page editor:
- Container: max-w-5xl → max-w-6xl (1024px → 1152px)
- Content: max-w-4xl → max-w-5xl (896px → 1024px)
- Toolbar: max-w-5xl → max-w-6xl (aligné)

Dialog editor:
- max-w 1800px → 2000px, 97vw → 98vw
- Hauteur 92vh → 94vh

Font editor:
- editor-body-size: 16px → 18px (lecture plus confortable)

memento-note/app/globals.css

@@ -985,7 +985,7 @@ html.font-system * {
 
   /* Editor body size — used for textarea and ProseMirror content */
   :root {
-    --editor-body-size: var(--user-font-size, 16px);
+    --editor-body-size: var(--user-font-size, 18px);
   }
 
   body {