chore: declare sideEffects allowlist for tree-shaking

Without the field, Rollup treats every module as potentially side-effectful
and cannot drop unused re-exports pulled through barrels. Audited all
import-time side effects: only CSS and the two bare side-effect imports
(router.ts, bindings.svelte.ts) must be preserved; module-level store
singletons ride their export usage and need no listing. Trims the bundle
~8 KB raw / ~2.4 KB gzip.

package.json

@@ -4,6 +4,11 @@
   "version": "0.0.1",
   "packageManager": "yarn@4.11.0",
   "type": "module",
+  "sideEffects": [
+    "*.css",
+    "**/router.ts",
+    "**/bindings.svelte.ts"
+  ],
   "scripts": {
     "dev": "vite",
     "build": "vite build",