feat: implement path prefix handling for OAuth and login redirects

lib/browser-navigation.ts

@@ -1,7 +1,51 @@
+import { locales } from '@/i18n/routing';
+
 export function replaceWindowLocation(url: string): void {
   if (typeof window === 'undefined') {
     return;
   }
 
   window.location.replace(url);
+}
+
+/**
+ * Returns the mount prefix from the current URL.
+ * When the app is served behind a reverse proxy at e.g. /bulwark,
+ * the browser sees /bulwark/en/login while Next.js sees /en/login.
+ *
+ * If a locale is supplied (e.g. from route params) it is used directly;
+ * otherwise the first path segment that matches a known locale is used.
+ *
+ * Returns '' when there is no prefix.
+ */
+export function getPathPrefix(locale?: string): string {
+  if (typeof window === 'undefined') return '';
+
+  const segments = window.location.pathname.split('/').filter(Boolean);
+
+  let localeIndex: number;
+  if (locale) {
+    localeIndex = segments.indexOf(locale);
+  } else {
+    localeIndex = segments.findIndex(s =>
+      (locales as readonly string[]).includes(s)
+    );
+  }
+
+  if (localeIndex <= 0) return '';
+  return '/' + segments.slice(0, localeIndex).join('/');
+}
+
+/**
+ * Extracts the locale from the current URL, skipping any mount prefix.
+ * Falls back to 'en' when no known locale segment is found.
+ */
+export function getLocaleFromPath(): string {
+  if (typeof window === 'undefined') return 'en';
+
+  const segments = window.location.pathname.split('/').filter(Boolean);
+  const locale = segments.find(s =>
+    (locales as readonly string[]).includes(s)
+  );
+  return locale || 'en';
 }