fix: resolve module resolution errors by replacing path aliases with relative imports

- Replace all @/ path aliases with proper relative imports and .js extensions
- Update @mollie/api-client peer dependency to support v4.x (^3.7.0 || ^4.0.0)
- Bump version to 0.1.5
- Ensure ESM compatibility for plugin distribution

Fixes module resolution error: "Cannot find package '@/collections'" when using
the plugin in external PayloadCMS projects.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/payload-billing",
-  "version": "0.1.4",
+  "version": "0.1.5",
   "description": "PayloadCMS plugin for billing and payment provider integrations with tracking and local testing",
   "license": "MIT",
   "type": "module",
@@ -106,7 +106,7 @@
     "vitest": "^3.1.2"
   },
   "peerDependencies": {
-    "@mollie/api-client": "^3.7.0",
+    "@mollie/api-client": "^3.7.0 || ^4.0.0",
     "payload": "^3.37.0",
     "stripe": "^18.5.0"
   },