feat: composer-sidebar slot + plugin-declared frame-src origins

components/email/email-composer.tsx

@@ -1100,8 +1100,13 @@ export function EmailComposer({
   };
 
   return (
+    <div className={cn("flex h-full bg-background", className)}>
+      <PluginSlot
+        name="composer-sidebar"
+        className="hidden md:flex shrink-0 h-full overflow-hidden border-r border-border"
+      />
     <div
-      className={cn("flex flex-col h-full bg-background relative", className)}
+      className="flex flex-col h-full bg-background relative flex-1 min-w-0"
       data-tour="composer"
       onDragEnter={handleDragEnter}
       onDragLeave={handleDragLeave}
@@ -1674,6 +1679,7 @@ export function EmailComposer({
         </div>
       )}
     </div>
+    </div>
   );
 }