fix(ui): use imported theme instead of undefined createTheme

createTheme was called without being imported, causing a runtime
ReferenceError on every page load. Use the already-imported theme
object from @homarr/ui which contains the full Mantine configuration.

apps/nextjs/src/app/[locale]/_client-providers/mantine.tsx

@@ -21,11 +21,7 @@ export const CustomMantineProvider = ({
       <MantineProvider
         defaultColorScheme={defaultColorScheme}
         colorSchemeManager={manager}
-        theme={createTheme({
-          primaryColor: "red",
-          autoContrast: true,
-          defaultRadius: "sm",
-        })}
+        theme={theme}
       >
         {children}
       </MantineProvider>