feat: server support contact display (MSC1929) and server notices UI

- Settings Help/About fetches /.well-known/matrix/support and displays
  admin contact + support page link (graceful 404 degradation)
- Server notice rooms (m.server_notice) now show a Warning badge in the
  room header and hide the message composer

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/room/RoomInput.tsx

@@ -603,6 +603,16 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
       });
     };
 
+    if (room.getType() === 'm.server_notice') {
+      return (
+        <div ref={ref} style={{ padding: config.space.S300, textAlign: 'center' }}>
+          <Text size="T300" priority="300">
+            This is a server notice room — you cannot send messages here.
+          </Text>
+        </div>
+      );
+    }
+
     return (
       <div ref={ref}>
         {selectedFiles.length > 0 && (