feat: add language switching to admin and merchant frontends

- Add cookie to ADMIN resolution chain (cookie → user_pref → "en")
- Add explicit MERCHANT resolution (cookie → user_pref → "fr")
- Add language selector dropdown to admin and merchant headers
- Add languageSelector() function to merchant init-alpine.js
- Add flag-icons CSS and i18n.js setup to merchant base template
- Add compact flag-based language selector to both login pages
- Make lang attribute dynamic on all base and login templates
- Pass current_language to login route template context
- Update architecture doc with ADMIN/MERCHANT resolution priorities

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/core/routes/pages/admin.py

@@ -57,7 +57,10 @@ async def admin_login_page(
     if current_user:
         return RedirectResponse(url="/admin/dashboard", status_code=302)
 
-    return templates.TemplateResponse("tenancy/admin/login.html", {"request": request})
+    return templates.TemplateResponse("tenancy/admin/login.html", {
+        "request": request,
+        "current_language": getattr(request.state, "language", "en"),
+    })
 
 
 @router.get("/select-platform", response_class=HTMLResponse, include_in_schema=False)

app/modules/core/routes/pages/merchant.py

@@ -67,7 +67,10 @@ async def merchant_login_page(
     if current_user:
         return RedirectResponse(url="/merchants/dashboard", status_code=302)
 
-    return templates.TemplateResponse("tenancy/merchant/login.html", {"request": request})
+    return templates.TemplateResponse("tenancy/merchant/login.html", {
+        "request": request,
+        "current_language": getattr(request.state, "language", "fr"),
+    })
 
 
 # ============================================================================