Preserve newlines when sending chat messages

The MentionEditor's serializeContent function was not handling hardBreak
nodes created by Shift+Enter. This caused newlines within messages to be
lost during serialization, even though the editor displayed them correctly.

Changes:
- Add hardBreak node handling in serializeContent
- Preserve newlines (\n) from Shift+Enter keypresses
- Ensure multi-line messages are sent with proper line breaks

With this fix and the previous Text.tsx fix, newlines are now properly:
1. Captured when typing (Shift+Enter creates hardBreak)
2. Preserved when sending (hardBreak serialized as \n)
3. Rendered when displaying (Text component renders \n as <br />)

src/components/editor/MentionEditor.tsx

@@ -584,6 +584,9 @@ export const MentionEditor = forwardRef<
             node.content?.forEach((child: any) => {
               if (child.type === "text") {
                 text += child.text;
+              } else if (child.type === "hardBreak") {
+                // Preserve newlines from Shift+Enter
+                text += "\n";
               } else if (child.type === "mention") {
                 const pubkey = child.attrs?.id;
                 if (pubkey) {