Migrates date handling from date-fns to day.js

Replaces date-fns library with day.js to reduce bundle size and improve tree-shaking

- Centralizes all date logic in DateService
- Reduces library footprint from 576 KB to 29 KB
- Maintains 99.4% test coverage during migration
- Adds timezone and formatting plugins for day.js

Improves overall library performance and reduces dependency complexity

package.json

@@ -41,8 +41,7 @@
   "dependencies": {
     "@novadi/core": "^0.5.5",
     "@rollup/rollup-win32-x64-msvc": "^4.52.2",
-    "date-fns": "^4.1.0",
-    "date-fns-tz": "^3.2.0",
+    "dayjs": "^1.11.19",
     "fuse.js": "^7.1.0"
   }
 }