fix(budget): correct VAT storage semantics for planned amounts

[steilerDev]

Summary

• Hotfix targeting main directly — this is an untracked fix for a data-correctness bug in VAT handling
• Semantic change: plannedAmount is now always stored as the net amount (as entered). VAT is applied at display/calculation time via a new effectivePlannedAmount() shared utility in @cornerstone/shared
• DB migration (0031_fix_vat_storage_semantics.sql) converts existing unit-mode includes_vat=false lines from VAT-inclusive back to net amounts to prevent double-counting after the semantic change

Root Cause

In direct pricing mode with "price includes VAT" checked, plannedAmount was divided by the VAT multiplier before storage. In unit mode, VAT was baked into the stored amount at save time. This caused double-VAT on display after the stored value was used as a base for further calculation.

Files Changed

File	Change
server/src/db/migrations/0031_fix_vat_storage_semantics.sql	New migration — converts existing stored amounts from net+VAT back to net
server/src/services/budgetOverviewService.ts	Remove save-time VAT application; use net amounts throughout
shared/src/types/budget.ts	Add effectivePlannedAmount() utility type/function
shared/src/index.ts	Export new shared utility
client/src/lib/budgetConstants.ts	Remove client-side save-time VAT adjustment
client/src/hooks/useBudgetSection.ts	Apply effectivePlannedAmount() at display time
client/src/components/budget/BudgetLineCard.tsx	Use display-time VAT via shared utility
client/src/components/budget/BudgetSection.tsx	Use display-time VAT via shared utility
client/src/pages/WorkItemDetailPage/WorkItemDetailPage.tsx	Remove save-time VAT division
client/src/pages/HouseholdItemDetailPage/HouseholdItemDetailPage.tsx	Remove save-time VAT division

Post-merge Action Required

After this PR merges to main, the hotfix must be cherry-picked to beta to keep the integration branch in sync.

Test Plan

• Quality Gates pass (typecheck + unit tests + build)
• E2E Gates pass (full Playwright suite — required for main target)
• Migration applies cleanly on a database with existing unit-mode budget lines
• Budget section totals display correct net vs. gross amounts after the fix

Co-Authored-By: Claude dev-team-lead (Sonnet 4.6) noreply@anthropic.com

[github-actions]

🎉 This PR is included in version 2.4.4 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 2.5.0-beta.3 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀