✨ Add labelFormat and header options to LocaleSwitcher

[damienlagae]

Problem

Apps carrying three or more locales cannot afford the full own-language name in the navbar — English / Français / Nederlands eats the space the menu items need — but the only alternative today, showLocaleName: false, leaves a bare flag with no textual cue at all.

So every app migrating off enabel/layout-bundle hand-rolls the same dropdown to get flag + short code + a "Change language" heading back. That is exactly what this component should provide.

There is also a rendering bug: flags were forced into a 1.5rem square. The cif:* set has no single aspect ratio (cif:nl is 2:1, cif:gb is 3:2), so the flag was letterboxed inside its own box and the border framed empty space rather than the flag:

Before	After
square box, flag shrunk and centred inside it	4:3 box, flag fills it, rounding hugs the flag

Changes

• labelFormat — name (default, unchanged), code (EN) or none (flag only).
• header — optional dropdown-header at the top of the menu, e.g. 'Change language'. Translate at the call site. Defaults to null, so nothing renders unless asked.
• Flags render rounded with a subtle border and carry the locale's own-language name as title, so labelFormat: 'none' stays accessible.
• Flags render in a fixed 4:3 box (1.5rem × 1.125rem) with preserveAspectRatio="xMidYMid slice". Both width and height are explicit: a host application's ux_icons.default_icon_attributes (commonly width/height: 1em) otherwise wins and forces the square back — width: auto is not enough.

{{ component('Enabel:Ux:LocaleSwitcher', {
    locales: enabled_locales,
    labelFormat: 'code',
    header: 'navigation.language.change'|trans,
}) }}

Renders 🇬🇧 EN in the navbar, and a dropdown headed Change language listing 🇫🇷 FR and 🇳🇱 NL.

Backward compatibility

showLocaleName keeps working — labelFormat is derived from it, so a call site passing false still resolves to none. An explicit labelFormat always wins. The property is marked @deprecated rather than removed, and its existing tests still pass untouched.

The only visual change for callers who pass nothing is the flag styling (correct aspect ratio, rounded, bordered, title); the label still defaults to the own-language name.

Verification

• vendor/bin/phpunit — 253 tests, 684 assertions, green (7 new LocaleSwitcher tests covering the derivation, the precedence rule, and the rejected values).
• php-cs-fixer and phpstan clean.
• Rendered in a real Symfony 8 app across /en, /fr and /nl: verified the emitted DOM carries title per locale, the rounded border border-dark-subtle classes, hrefs /fr and /nl, the heading, and a 24×18 flag box.

Drive-by

The docs documented the parameter as show_locale_name; the component has always read showLocaleName.