fix(build): use regex pattern for three.js externalization in rsplat

The string pattern "three/addons/" didn't match "three/examples/jsm/..."
imports, causing vite build to fail on a clean Docker build.

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

vite.config.ts

@@ -1023,7 +1023,7 @@ export default defineConfig({
 								fileName: () => "folk-splat-viewer.js",
 							},
 							rollupOptions: {
-								external: ["three", "three/addons/", "@mkkellogg/gaussian-splats-3d"],
+								external: [/^three($|\/)/, "@mkkellogg/gaussian-splats-3d"],
 								output: {
 									entryFileNames: "folk-splat-viewer.js",
 								},