Dev-Umgebung: Kompatibilität für PHP 8.3 + Smarty 4.5.6

- Smarty 4.1.1 → 4.5.6 (behebt dynamic property deprecations)
- Core-Klassen: #[\AllowDynamicProperties] für Admin_role, base, Config,
  Customer, Customer_group, CustomerGroups, Item, Structure, website
- website.class.php: counts[parent_id] initialisieren vor ++ (PHP 8.1)
- layout.class.php: HTTP_ACCEPT_LANGUAGE mit isset-Guard
- website_init.php: session_status()-Check vor session_start
- .htaccess: HTTPS-Redirect via X-Forwarded-Proto (statt SERVER_PORT)
- themes/easyshop_advanced/media/: Parent-Theme-Assets nachgezogen
- .gitignore: smarty.4.1.1.bak ausschließen

libs/smarty/docs/programmers/api-variables/variable-cache-modified-check.md

@@ -0,0 +1,12 @@
+\$cache\_modified\_check {#variable.cache.modified.check}
+========================
+
+If set to TRUE, Smarty will respect the If-Modified-Since header sent
+from the client. If the cached file timestamp has not changed since the
+last visit, then a `'304: Not Modified'` header will be sent instead of
+the content. This works only on cached content without
+[`{insert}`](#language.function.insert) tags.
+
+See also [`$caching`](#variable.caching),
+[`$cache_lifetime`](#variable.cache.lifetime), and the [caching
+section](#caching).