Initial commit: Voiced invoice desktop app

Offline-first invoicing app for Test Vendor, built by Bongbetic.

- Tauri v2 (Rust) + React/TypeScript + IBM Carbon Design System UI
- SQLite (rusqlite) storage with a persistent invoice-number series
- Guided first-run setup, pre-filled vendor/bank details
- Fixed-amount or rate x quantity line items (per sec/min/hour/session/unit)
- GST as CGST+SGST or IGST, Indian amount-in-words
- Optional signature, with a computer-generated note as fallback
- Print-ready PDF export via @react-pdf/renderer
- Windows cross-compile via cargo-xwin (NSIS installer)

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

src/lib/invoice.ts

@@ -0,0 +1,54 @@
+import { round2 } from "./format";
+import type { InvoiceItem, TaxType } from "./types";
+
+/** 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);
+}
+
+export interface Totals {
+  subtotal: number;
+  discount: number;
+  taxable: number;
+  cgst: number;
+  sgst: number;
+  igst: number;
+  taxTotal: number;
+  total: number;
+}
+
+export function computeTotals(
+  items: InvoiceItem[],
+  discount: number,
+  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);
+
+  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);
+  } else if (taxType === "igst") {
+    igst = round2((taxable * taxRate) / 100);
+  }
+  const taxTotal = round2(cgst + sgst + igst);
+
+  return {
+    subtotal,
+    discount: discountValue,
+    taxable,
+    cgst,
+    sgst,
+    igst,
+    taxTotal,
+    total: round2(taxable + taxTotal),
+  };
+}