fix(i18n): default startup language to English

First-visit users now always land on the English UI regardless of
navigator.language. Arabic remains one click away via the toggle, and
the user's explicit pick is persisted to localStorage (encoach-lang)
and honoured on every subsequent load.

- src/i18n/index.ts: set lng: "en", drop navigator/htmlTag from the
  detector order so an Arabic-locale browser no longer silently boots
  the UI in Arabic before the user has chosen.
- src/lib/api-client.ts: mirror the same policy for the Accept-Language
  header sent to the backend, so AI-generated content stays English on
  first visit instead of echoing the browser locale.

Made-with: Cursor

src/i18n/index.ts

@@ -5,10 +5,16 @@
  * they go through type-checking and tree-shaking, and so non-translators
  * can't ship a broken JSON file that blocks the build.
  *
- * Language detection order:
- *   1. localStorage key ``encoach-lang`` (explicit user pick)
- *   2. ``navigator.language``
- *   3. fallback ``en``
+ * Language selection order (product decision: English is the default UI
+ * language at startup; Arabic is opt-in via the language toggle):
+ *   1. localStorage key ``encoach-lang`` (explicit user pick, persisted)
+ *   2. ``en`` — fallback used on first visit, regardless of browser locale
+ *
+ * We intentionally do **not** read ``navigator.language`` anymore: an
+ * Arabic-locale browser should still land on the English UI the first time
+ * a user visits, because the QA team and the product manager both sign off
+ * against the English baseline. Users that actively want Arabic flip the
+ * toggle once and the preference is remembered from then on.
  *
  * RTL handling: whenever the active language switches to one of RTL_LANGS,
  * we flip ``document.documentElement.dir`` so Tailwind's RTL utilities +
@@ -31,6 +37,8 @@ export const SUPPORTED_LANGS = [
 
 export type SupportedLang = (typeof SUPPORTED_LANGS)[number]["code"];
 
+export const DEFAULT_LANG: SupportedLang = "en";
+
 i18n
   .use(LanguageDetector)
   .use(initReactI18next)
@@ -39,11 +47,16 @@ i18n
       en: { translation: en },
       ar: { translation: ar },
     },
-    fallbackLng: "en",
+    // Explicit default — english is the baseline UI language at startup.
+    lng: DEFAULT_LANG,
+    fallbackLng: DEFAULT_LANG,
     supportedLngs: SUPPORTED_LANGS.map((l) => l.code),
     interpolation: { escapeValue: false },
     detection: {
-      order: ["localStorage", "navigator", "htmlTag"],
+      // Only honour the user's saved pick. Navigator/htmlTag detection was
+      // removed so Arabic-locale browsers don't silently boot the UI in
+      // Arabic before the user has chosen.
+      order: ["localStorage"],
       caches: ["localStorage"],
       lookupLocalStorage: "encoach-lang",
     },

src/lib/api-client.ts

@@ -131,15 +131,17 @@ function refreshOnce(): Promise<boolean> {
 }
 
 function getCurrentLanguage(): string {
+  // Mirrors the i18n bootstrap in src/i18n/index.ts: the user's explicit
+  // pick wins; everything else falls back to English. We intentionally do
+  // not consult navigator.language here so AI-generated content stays in
+  // English on first visit (matching the UI default) until the user flips
+  // the language toggle.
   try {
     const stored = localStorage.getItem("encoach-lang");
     if (stored) return stored;
   } catch {
     // localStorage unavailable (SSR, sandboxing, etc.)
   }
-  if (typeof navigator !== "undefined" && navigator.language) {
-    return navigator.language.split("-")[0];
-  }
   return "en";
 }