fix: storefront login 403, cookie path, double-storefront URLs, and auth redirects

- Extract store/platform context from Referer header for storefront API requests
  (StoreContextMiddleware and PlatformContextMiddleware) so login POST works in
  dev mode where API paths lack /platforms/{code}/ prefix
- Set customer token cookie path to "/" for cross-route compatibility
- Fix double storefront in URLs: replace {{ base_url }}storefront/ with {{ base_url }}
  across all 24 storefront templates
- Fix auth error redirect to include platform prefix and use store_code
- Update seed script to output correct storefront login URLs
- Add 20 new unit tests covering all fixes; fix 9 pre-existing test failures

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

tests/unit/middleware/test_language.py

@@ -132,6 +132,7 @@ class TestLanguageDispatchStore:
         mock_resolve.assert_called_once_with(
             user_preferred="de",
             store_dashboard="fr",
+            cookie_language=None,
         )
         assert request.state.language == "de"
 
@@ -154,6 +155,7 @@ class TestLanguageDispatchStore:
         mock_resolve.assert_called_once_with(
             user_preferred=None,
             store_dashboard=None,
+            cookie_language=None,
         )
 
 
@@ -356,6 +358,7 @@ class TestLanguageHelpers:
             max_age=60 * 60 * 24 * 365,
             httponly=False,
             samesite="lax",
+            path="/",
         )
         assert result is response
 
@@ -372,7 +375,7 @@ class TestLanguageHelpers:
         response = Mock(spec=Response)
         result = delete_language_cookie(response)
 
-        response.delete_cookie.assert_called_once_with(key="lang")
+        response.delete_cookie.assert_called_once_with(key="lang", path="/")
         assert result is response
 
     def test_get_user_language_from_token_with_pref(self):