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

frontend/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";
 }