feat: add year 2026 for 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 adds support for tax year 2026 in the gross-to-net salary calculator by implementing the 2026 income tax calculation logic and updating the validation schema to accept '2026' as a valid accounting year.

• Added new Lohnsteuer2026 income tax calculation module
• Updated schema validation to include 2026 as valid accounting year
• Added social insurance threshold value for 2026

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/utils/Lohnsteuer/2026.ts	New income tax calculation implementation for 2026 with German tax law parameters and formulas
src/utils/Lohnsteuer/index.ts	Registered 2026 tax year in income tax classes mapping
src/calculators/gross-to-net.ts	Added 2026 to valid year enum and defined social threshold constant
test/calculators/gross-to-net.test.ts	Added test case for 2026 tax year calculations
test/calculators/snapshots/gross-to-net.test.ts.snap	Generated snapshot for 2026 test expectations

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[johannschopplich]

Schön! 👏