Make Enter insert newline on mobile devices

On mobile devices, pressing Enter now inserts a newline (hardBreak) instead
of submitting the message. This provides better UX since mobile keyboards
don't have easy access to Shift+Enter for multiline input.

Behavior:
- Desktop: Enter submits, Shift+Enter inserts newline (unchanged)
- Mobile: Enter inserts newline, Cmd/Ctrl+Enter submits
- Mobile detection: Uses touch support API (ontouchstart or maxTouchPoints)

Users can still submit messages on mobile using:
1. The Send button (primary method)
2. Ctrl+Enter keyboard shortcut (if available)

src/components/editor/MentionEditor.tsx

@@ -667,6 +667,9 @@ export const MentionEditor = forwardRef<
 
     // Build extensions array
     const extensions = useMemo(() => {
+      // Detect mobile devices (touch support)
+      const isMobile = "ontouchstart" in window || navigator.maxTouchPoints > 0;
+
       // Custom extension for keyboard shortcuts (runs before suggestion plugins)
       const SubmitShortcut = Extension.create({
         name: "submitShortcut",
@@ -677,10 +680,16 @@ export const MentionEditor = forwardRef<
               handleSubmitRef.current(editor);
               return true;
             },
-            // Plain Enter submits (Shift+Enter handled by hardBreak for newlines)
+            // Plain Enter behavior depends on device
             Enter: ({ editor }) => {
-              handleSubmitRef.current(editor);
-              return true;
+              if (isMobile) {
+                // On mobile, Enter inserts a newline (hardBreak)
+                return editor.commands.setHardBreak();
+              } else {
+                // On desktop, Enter submits the message
+                handleSubmitRef.current(editor);
+                return true;
+              }
             },
           };
         },