The original CSS scoped .flag to .locale-switcher__option only, so the
flag SVG inside the <summary> trigger rendered at 0x0 (intrinsic svg
defaults). Add a .locale-switcher .flag rule so the closed trigger
visibly shows a 24x16 flag.
Playwright recheck: trigger 52x44, flag 24x16 (matches previous inline
flags). 141/141 PHPUnit green.