fix(frontend): always default first-time visitors to Persian

Drop Accept-Language detection from the locale middleware. A first
visit (no NEXT_LOCALE cookie) now always serves the default locale
(fa-IR) instead of guessing from the browser. The user's last explicit
choice, persisted in the cookie by the language switcher, remains the
sole signal on subsequent visits.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/middleware.ts

@@ -1,19 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { locales, defaultLocale, isLocale, LOCALE_COOKIE, type Locale } from '@/i18n/config';
 
-// Parse the Accept-Language header into an ordered list of base language codes.
-function parseAcceptLanguage(header: string | null): string[] {
-  if (!header) return [];
-  return header
-    .split(',')
-    .map((part) => {
-      const [tag, q] = part.trim().split(';q=');
-      return { tag: tag.trim().toLowerCase(), q: q ? parseFloat(q) : 1 };
-    })
-    .sort((a, b) => b.q - a.q)
-    .map((x) => x.tag);
-}
-
 // Host-based separation between the marketing landing site and the
 // authenticated panel. Configured at runtime (server-side) so the same
 // frontend image can serve both hosts. When PANEL_HOST is unset (local dev),
@@ -42,20 +29,14 @@ function hostOf(request: NextRequest): string {
 }
 
 function detectLocale(request: NextRequest): Locale {
-  // 1. Explicit choice via cookie
+  // Honour the user's last explicit choice, persisted in a cookie by the
+  // language switcher. On the very first visit (no cookie) we always serve the
+  // default locale (fa-IR) rather than guessing from Accept-Language, so the
+  // browser's language preference never overrides the default. From then on,
+  // the user's last selection is what determines the language.
   const cookieLocale = request.cookies.get(LOCALE_COOKIE)?.value;
   if (isLocale(cookieLocale)) return cookieLocale;
 
-  // 2. Browser preference via Accept-Language
-  const accepted = parseAcceptLanguage(request.headers.get('accept-language'));
-  for (const tag of accepted) {
-    const exact = locales.find((l) => l.toLowerCase() === tag);
-    if (exact) return exact;
-    const base = locales.find((l) => l.toLowerCase().split('-')[0] === tag.split('-')[0]);
-    if (base) return base;
-  }
-
-  // 3. Fallback
   return defaultLocale;
 }