feat: adjust "Arbeit­ge­ber­zu­schuss" for each year in gross-to-net

[alex-rie]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSDoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[Copilot]

Pull request overview

This PR adjusts the maximum employer subsidy ("Arbeitgeberzuschuss") for private health insurance to use year-specific values instead of a simple 2019 vs. post-2019 split. The implementation adds a lookup table mapping accounting years 2019-2026 to their respective subsidy amounts, with a fallback value for unknown years.

Changes:

• Replaced the conditional logic for employer subsidy calculation with a year-indexed lookup object
• Added specific maximum employer grant values for years 2019-2026
• Updated test snapshots to reflect the corrected calculations for the default test year

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/calculators/gross-to-net.ts	Replaced binary year check with comprehensive year-to-subsidy mapping table
test/calculators/__snapshots__/gross-to-net.test.ts.snap	Updated snapshot values to match recalculated employer subsidy amounts

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[johannschopplich]

LGTM! 🙂