fix(settings): keep Steam ID as string and store the new token

Number("76561198005679788") is 76561198005679790 in JavaScript, so the wrong
Steam ID was saved. The Steam ID now stays a 17-digit string. Theme and
language saves no longer send steamId 0. After a save the new token from Auth
replaces the old one.

Add Vitest and run it in CI.

package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run"
   },
   "dependencies": {
     "@hookform/resolvers": "^3.10.0",
@@ -26,6 +27,7 @@
     "autoprefixer": "~10.4.27",
     "postcss": "~8.5.8",
     "tailwindcss": "~4.2.2",
-    "vite": "^8.0.1"
+    "vite": "^8.0.1",
+    "vitest": "^5.0.1"
   }
 }