fix: exclude test files from webpack TypeScript checking

Exclude src/**/*.test.* and src/**/*.spec.* from tsconfig.json so
webpack dev server doesn't type-check test files. Also narrow the
site.config glob from site.config.*.tsx to site.config.dev.tsx to
avoid including site.config.test.tsx.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: arbrandes

tsconfig.json

@@ -14,6 +14,10 @@
     "babel.config.js",
     "eslint.config.js",
     "jest.config.js",
-    "site.config.*.tsx"
+    "site.config.dev.tsx"
+  ],
+  "exclude": [
+    "src/**/*.test.*",
+    "src/**/*.spec.*"
   ]
 }