fix: return 404 on missing assets (@fehmer)

Author: fehmer

frontend/firebase.json

@@ -4,27 +4,11 @@
     "ignore": ["firebase.json", "**/.*", "**/node_modules/**"],
     "rewrites": [
       {
-        "source": "/privacy-policy",
-        "destination": "/privacy-policy.html"
+        "source": "/@(js|about|challenges|css|fonts|funbox|images|languages|layouts|quotes|sound|themes|webfonts|webfonts-preview)/**",
+        "destination": "/404.html"
       },
       {
-        "source": "/terms-of-service",
-        "destination": "/terms-of-service.html"
-      },
-      {
-        "source": "/security-policy",
-        "destination": "/security-policy.html"
-      },
-      {
-        "source": "/adtest",
-        "destination": "/adtest.html"
-      },
-      {
-        "source": "/test",
-        "destination": "/index.html"
-      },
-      {
-        "source": "!/@(js|about|challenges|css|fonts|funbox|images|languages|layouts|quotes|sound|themes|webfonts)/**",
+        "source": "**",
         "destination": "/index.html"
       }
     ],