fix: upgrade frontend to Svelte 5 + Vite 6 for dependency compatibility

Svelte 4 conflicts with latest @sveltejs/kit peer requirements.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -8,9 +8,9 @@
     "preview": "vite preview"
   },
   "devDependencies": {
-    "@sveltejs/adapter-node": "^2.0.0",
+    "@sveltejs/adapter-node": "^5.0.0",
     "@sveltejs/kit": "^2.0.0",
-    "svelte": "^4.0.0",
-    "vite": "^5.0.0"
+    "svelte": "^5.0.0",
+    "vite": "^6.0.0"
   }
 }