Pos_Invoicing

Description:
Build an offline-first Windows POS system that supports B2C and B2B sales with correct GST calculation (CGST/SGST vs IGST), local stock updates, and reliable sync with the server.

The POS should allow invoice creation, customer handling, stock deduction, and return processing completely offline. All posted documents (sales and returns) must be immutable and stored as exact snapshots. Sync must be idempotent to avoid duplicates and ensure consistency between POS and server.

System should also handle GST at item level, determine tax type based on state comparison, and support day-close reporting for daily operations.

Acceptance Criteria:

POS works fully offline for sales and returns
Store binding required before billing
Correct GST applied (intra → CGST+SGST, inter → IGST)
Invoice and return documents are immutable after posting
Stock updates correctly (sale → decrease, return → increase)
Sync to server is idempotent (no duplicate invoices)
Invoice numbering unique per GSTIN + financial year
Return linked to original invoice with correct GST reversal
Day close report matches local transactions
Server stores exact snapshot received from POS