Exclude test files from frontend production TypeScript build

Test files use Vitest globals (vi, describe, it, expect) which aren't
available to tsc during production builds. Exclude __tests__ dirs and
*.test.* / *.spec.* files from tsconfig.app.json so tsc -b succeeds.

frontend/tsconfig.app.json

@@ -34,5 +34,10 @@
   "include": [
     "src",
     "public/HexgridHeatmap.js",
+  ],
+  "exclude": [
+    "src/**/__tests__/**",
+    "src/**/*.test.*",
+    "src/**/*.spec.*"
   ]
 }