feat!: Refactor billing module and add checkout and plans components

[roble]

This pull request refactors the billing module's JavaScript code organization and updates a copyright statement. The main change is moving the billing module setup and related imports from resources/js/app.ts to a new file, resources/js/vue/app.ts, and updating the export in app.ts to re-export from this new location. Additionally, the stylesheet import path is now more explicit.

Code organization and modularization:

• Moved billing module setup logic, icon registrations, and after-mount logic from resources/js/app.ts to a new dedicated file, resources/js/vue/app.ts, improving code organization and separation of concerns. ([[1]](https://github.com/saucebase-dev/billing/pull/22/files#diff-743ef4ebcd98b1f3998b49616e0389aa03b0e0e007101d73bc6aa07b09c783a4L1-R1), [[2]](https://github.com/saucebase-dev/billing/pull/22/files#diff-29313a9a39e44564e58410a58b8b3c5a6cda9f9cabf5539440213ca433996843R1-R24))
• Updated the import path for the billing module stylesheet to @modules/billing/resources/css/style.css for clarity and modularity. ([resources/js/vue/app.tsR1-R24](https://github.com/saucebase-dev/billing/pull/22/files#diff-29313a9a39e44564e58410a58b8b3c5a6cda9f9cabf5539440213ca433996843R1-R24))
• Changed resources/js/app.ts to re-export all exports from resources/js/vue/app.ts, simplifying the main app entry point. ([resources/js/app.tsL1-R1](https://github.com/saucebase-dev/billing/pull/22/files#diff-743ef4ebcd98b1f3998b49616e0389aa03b0e0e007101d73bc6aa07b09c783a4L1-R1))

License update:

• Updated the copyright year in the LICENSE file from 2025 to 2026. ([LICENSEL3-R3](https://github.com/saucebase-dev/billing/pull/22/files#diff-c693279643b8cd5d248172d9c22cb7cf4ed163a3c98c8a3f69c2717edd3eacb7L3-R3))