Fix myroomnick and cosmetics settings not updating (#805)

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### Description

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relevant motivation and context. List any dependencies that are required
for this change. -->

Fix myroomnick and cosmetics settings not updating
Fixes #787 

#### Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update

### Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings

### AI disclosure:

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Author: 7w1

.changeset/fix-myroomnick.md

@@ -0,0 +1,5 @@
+---
+default: patch
+---
+
+Fix `/myroomnick` and room cosmetics display name not updating name.

src/app/hooks/useCommands.ts

@@ -497,7 +497,19 @@ export const useCommands = (mx: MatrixClient, room: Room): CommandRecord => {
             ?.getStateEvents(EventType.RoomMember, mx.getSafeUserId());
           const content = mEvent?.getContent<RoomMemberEventContent>();
           if (!content) return;
-          await mx.sendStateEvent(room.roomId, EventType.RoomMember, content, mx.getSafeUserId());
+          const updatedContent: RoomMemberEventContent = { ...content };
+          const withDisplay = updatedContent as RoomMemberEventContent & { displayname?: string };
+          if (nick == null || nick === '') {
+            delete withDisplay.displayname;
+          } else {
+            withDisplay.displayname = nick;
+          }
+          await mx.sendStateEvent(
+            room.roomId,
+            EventType.RoomMember,
+            updatedContent,
+            mx.getSafeUserId()
+          );
         },
       },
       [Command.AddPerMessageProfileToAccount]: {