refactor: extract reusable library

- move reusable domain, data, state, ui code into src/lib
- update host screens to consume new library exports
- document architecture and configure path aliases
- bump astro integration dependencies for compatibility

Refs #30

package.json

@@ -12,8 +12,8 @@
     "test:replay": "playwright test"
   },
   "dependencies": {
-    "@astrojs/preact": "^4.1.0",
-    "astro": "^5.9.0",
+    "@astrojs/preact": "^4.1.3",
+    "astro": "^5.15.5",
     "preact": "^10.26.8"
   },
   "devDependencies": {