Add Scandinavian i18n (da/sv/nb) via DeepL pipeline.

Integrate new locale bundles, language cycling in the UI, SEO hreflang tags, and localized beta flyer HTML variants with scripts for batch translation and key validation.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/utils/i18nLanguages.test.ts

@@ -0,0 +1,21 @@
+import { describe, expect, it } from 'vitest'
+import { getNextLanguage, normalizeAppLanguage, SUPPORTED_LANGUAGES } from './i18nLanguages.js'
+
+describe('i18nLanguages', () => {
+  it('normalizes regional tags to supported base codes', () => {
+    expect(normalizeAppLanguage('de-DE')).toBe('de')
+    expect(normalizeAppLanguage('nb-NO')).toBe('nb')
+    expect(normalizeAppLanguage('xx')).toBe('en')
+  })
+
+  it('cycles through all supported languages', () => {
+    let current: string = 'de'
+    const seen = new Set<string>()
+    for (let i = 0; i < SUPPORTED_LANGUAGES.length; i++) {
+      seen.add(current)
+      current = getNextLanguage(current)
+    }
+    expect(seen.size).toBe(SUPPORTED_LANGUAGES.length)
+    expect(current).toBe('de')
+  })
+})