The new content-language options were hand-duplicated across request
validation, the UI picker, and homepage detection, while the brand
analyzer's structured-output enum and the AI image prompt's language
name still only knew about en/pt-BR/es. That left autofill unable to
detect the new languages and made image text fall back to English for
them.
Introduce App\Enums\Workspace\ContentLanguage as the single source of
truth and derive every site from it:
- Store/UpdateWorkspaceRequest validate against ContentLanguage::values()
- BrandAnalyzer's language enum uses ContentLanguage::values()
- AiImageClient::languageName() resolves via the enum's englishName()
- HomepageMetaExtractor detects through ContentLanguage::fromHtmlLang()
- BrandForm consumes availableContentLanguages from the backend, like
availableFonts/availableImageStyles, instead of a hardcoded list
Also fix two labels: nl "Nederlandse" -> "Nederlands", zh -> "中文".
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