Add test suite with 132 unit tests across all modules

Covers DesignState (40 tests), HouseRenderer (19), InteractionManager (24),
ThemeManager (8), ExportManager (11), and CatalogPanel (30). Uses vitest
with THREE.js mocks for browser-free testing.

package.json

@@ -0,0 +1,12 @@
+{
+  "name": "house-design",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "devDependencies": {
+    "vitest": "^3.0.0"
+  }
+}