Implement tax handling for branches by adding tax percent to regions, updating product sell costs, and enhancing UI for tax summaries in BranchView

backend/README_TAX.md

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+# Falukant Tax Migration & Configuration
+
+This project now supports a per-region sales tax (`tax_percent`) for Falukant.
+
+Migration
+- A SQL migration was added: `backend/migrations/20260101000000-add-tax-percent-to-region.cjs`.
+- It adds `tax_percent` numeric NOT NULL DEFAULT 7 to `falukant_data.region`.
+
+Runtime configuration
+- If you want taxes to be forwarded to a treasury account, set environment variable `TREASURY_FALUKANT_USER_ID` to a valid `falukant_user.id`.
+- If `TREASURY_FALUKANT_USER_ID` is not set, taxes will be calculated and currently not forwarded to any account.
+
+Implementation notes
+- Backend service `sellProduct` and `sellAllProducts` now compute tax per-region and credit net to seller and tax to treasury (if configured).
+- Tax arithmetic uses rounding to 2 decimals. The current implementation performs two separate DB calls (seller, treasury). For strict ledger atomicity consider implementing DB-side booking.
+
+Cumulative tax behavior
+- The system now sums `tax_percent` from the sale region and all ancestor regions (recursive up the region tree). This allows defining different tax rates on up to 6 region levels and summing them for final tax percent.
+- To avoid reducing seller net by taxes, sale prices are inflated by factor = 1 / (1 - cumulativeTax/100). This way the seller receives the original net and the tax is collected separately.
+
+Testing
+- After running the migration, test with a small sale and verify `falukant_log.moneyflow` entries for seller and treasury.
+