The nav previously showed 4 inline flag buttons (DE/EN/UK/RU) on desktop
and a details-based dropdown on mobile. Martin asked for one dropdown with
a trigger the size of a single flag, and the 4 inline flags to go away.
- LocaleSwitcher: render a single <details class='locale-switcher'>
everywhere; trigger is one flag + tiny caret; menu lists all 4 with labels.
- Drop the 4-inline <ul> and the locale-switcher-mobile duplicate.
- CSS: replace both blocks with one compact dropdown (flag-sized trigger,
44px touch target via padding, scrolled/transparent-nav variants).
- Tests: assert 4 menu options, 5 flag SVGs, single <details> dropdown,
active locale is a <span aria-current>, others are <a> with hreflang.
- 141/141 PHPUnit green.
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