Add issue/cancel lifecycle, GST derivation and integer-paise totals

issue_invoice allocates the number, freezes vendor and bank details and
stores server-computed totals in one transaction. Issued invoices are
cancelled, not deleted. Tax heads derive from supplier state and place of
supply (a disagreeing choice is rejected); unregistered suppliers issue a
plain Invoice. Adds GSTIN checksum validation, canonical India Compliance
state list, server-side drafts, series validation, relative and
magic-byte-checked asset paths, and GST settings. PDF re-exports use the
frozen vendor snapshot.

src/lib/invoice.ts

@@ -1,12 +1,23 @@
-import { round2 } from "./format";
 import type { InvoiceItem, TaxType } from "./types";
 
+// The preview must show exactly what Rust stores (src-tauri/src/gst.rs), so money is
+// computed in integer paise with round-half-up, the same way.
+
+const toPaise = (rupees: number) => Math.round((Number.isFinite(rupees) ? rupees : 0) * 100);
+
+function linePaise(item: InvoiceItem): number {
+  if (item.mode === "rate") {
+    return Math.round(toPaise(item.rate || 0) * (item.quantity || 0));
+  }
+  return toPaise(item.amount || 0);
+}
+
+/** Round-half-up division for non-negative integers. */
+const divRound = (num: number, den: number) => Math.floor((num + den / 2) / den);
+
 /** Amount for a single line: a fixed total, or rate × quantity. */
 export function computeLineAmount(item: InvoiceItem): number {
-  if (item.mode === "rate") {
-    return round2((item.rate || 0) * (item.quantity || 0));
-  }
-  return round2(item.amount || 0);
+  return linePaise(item) / 100;
 }
 
 export interface Totals {
@@ -26,29 +37,30 @@ export function computeTotals(
   taxType: TaxType,
   taxRate: number,
 ): Totals {
-  const subtotal = round2(items.reduce((sum, item) => sum + computeLineAmount(item), 0));
-  const discountValue = round2(Math.min(Math.max(discount || 0, 0), subtotal));
-  const taxable = round2(subtotal - discountValue);
+  const subtotal = items.reduce((sum, item) => sum + linePaise(item), 0);
+  const discountPaise = Math.min(Math.max(toPaise(discount), 0), Math.max(subtotal, 0));
+  const taxable = subtotal - discountPaise;
+  const rateBp = Math.round((Number.isFinite(taxRate) ? taxRate : 0) * 100);
 
   let cgst = 0;
   let sgst = 0;
   let igst = 0;
   if (taxType === "cgst_sgst") {
-    cgst = round2((taxable * (taxRate / 2)) / 100);
-    sgst = round2((taxable * (taxRate / 2)) / 100);
+    cgst = divRound(taxable * rateBp, 20_000);
+    sgst = cgst;
   } else if (taxType === "igst") {
-    igst = round2((taxable * taxRate) / 100);
+    igst = divRound(taxable * rateBp, 10_000);
   }
-  const taxTotal = round2(cgst + sgst + igst);
+  const taxTotal = cgst + sgst + igst;
 
   return {
-    subtotal,
-    discount: discountValue,
-    taxable,
-    cgst,
-    sgst,
-    igst,
-    taxTotal,
-    total: round2(taxable + taxTotal),
+    subtotal: subtotal / 100,
+    discount: discountPaise / 100,
+    taxable: taxable / 100,
+    cgst: cgst / 100,
+    sgst: sgst / 100,
+    igst: igst / 100,
+    taxTotal: taxTotal / 100,
+    total: (taxable + taxTotal) / 100,
   };
 }