Fix dev user switcher tooltip covering dropdown options

The MUI Tooltip stayed anchored while the Select menu was open (the select
keeps focus) and overlapped the first user options. Use a native title
attribute instead, which browsers suppress while the popup is open.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -43,8 +43,10 @@ const DEV_USERS = [
 ];
 
 function DevUserSwitcher() {
+  // Native title (not a MUI Tooltip): a Tooltip stays anchored while the
+  // dropdown is open and covers the first menu items.
   return (
-    <Tooltip title="AUTH_MODE=dev — switch the simulated user">
+    <Box component="span" title="AUTH_MODE=dev — switch the simulated user">
       <Select
         size="small"
         value={getDevUser()}
@@ -66,7 +68,7 @@ function DevUserSwitcher() {
           </MenuItem>
         ))}
       </Select>
-    </Tooltip>
+    </Box>
   );
 }