✨ refactor: streamline SSR setup and remove Babel configuration

- Updated import path for app.server module in SSR hook.
- Removed babel.config.server.json as Babel is no longer needed for async/await transformation.
- Adjusted esbuild configuration to target ESNext and modified output paths.
- Enhanced App.svelte to handle initial content loading during SSR.
- Updated SSR script to load messages synchronously before rendering.
- Simplified build:server script in package.json by removing Babel step.

frontend/src/App.svelte

@@ -213,8 +213,21 @@
         loading = false
     }
 
+    if (typeof window === "undefined") {
+        ;(() => {
+            const initialPath = url.split("?")[0] || "/"
+            const initialLang = extractLanguageFromPath(initialPath) || DEFAULT_LANGUAGE
+            const initialRoutePath = stripLanguageFromPath(initialPath) || "/"
+            loadContent(initialLang, initialRoutePath)
+        })()
+    }
+
     // Re-load content when path or language changes
     $effect(() => {
+        if (typeof window === "undefined") {
+            return
+        }
+
         const lang = $currentLanguage
         const routePath = stripLanguageFromPath($location.path)
         loadContent(lang, routePath || "/")