feat(frontend): handle no brain selection (#2932)

# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context.

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

frontend/app/chat/[chatId]/components/ActionsBar/components/ChatInput/components/ChatEditor/Editor/Editor.tsx

@@ -63,7 +63,24 @@ export const Editor = ({
   return (
     <EditorContent
       className="w-full caret-accent"
-      onKeyDown={(event) => void submitOnEnter(event)}
+      onKeyDown={(event) => {
+        if (event.key === "Enter" && !event.shiftKey && !currentBrain) {
+          event.preventDefault();
+          const lastChar = editor?.state.doc.textBetween(
+            editor.state.selection.$from.pos - 1,
+            editor.state.selection.$from.pos,
+            undefined,
+            "\ufffc"
+          );
+          if (lastChar === " ") {
+            editor?.chain().insertContent("@").focus().run();
+          } else {
+            editor?.chain().insertContent(" @").focus().run();
+          }
+        } else {
+          submitOnEnter(event);
+        }
+      }}
       editor={editor}
     />
   );

frontend/app/chat/[chatId]/components/ActionsBar/components/ChatInput/components/ChatEditor/Editor/hooks/useCreateEditorState.ts

@@ -16,7 +16,7 @@ export const useCreateEditorState = (placeholder?: string) => {
   const { BrainMention, items } = useBrainMention();
   const { remainingCredits } = useUserSettingsContext();
 
-  const PreventEnter = Extension.create({
+  const PreventNewline = Extension.create({
     addKeyboardShortcuts: () => {
       return {
         Enter: () => true,
@@ -35,7 +35,7 @@ export const useCreateEditorState = (placeholder?: string) => {
         preserveWhitespace: "full",
       },
       extensions: [
-        PreventEnter,
+        PreventNewline,
         Placeholder.configure({
           showOnlyWhenEditable: true,
           placeholder: placeholder ?? t("actions_bar_placeholder"),