Replace the HTML preview with a worker-rendered PDF preview

Rendering moves into a module Web Worker behind a latest-wins RenderClient
with a watchdog and a handshake-based main-thread fallback. The preview
shows the real PDF through pdf.js at 2x with zoom, page indicator, audit
issues popover and a focus modal. A4/Letter and margin controls are frozen
into each invoice at issue and can be saved as the default (migration M3).
react-pdf and pdf.js are lazy-loaded; the main chunk shrinks from 1.8 MB
to 0.5 MB. The HTML twin and its styles are removed.

src/lib/previewInvoice.ts

@@ -0,0 +1,98 @@
+import type { BankAccount, Invoice, InvoiceItem, Settings, TaxType } from "./types";
+import { computeLineAmount, computeTotals } from "./invoice";
+import { amountInWords } from "./numberToWords";
+
+/** The form fields the preview reads. */
+export interface PreviewForm {
+  invoiceDate: string;
+  dueDate: string;
+  clientId: number | null;
+  clientName: string;
+  clientAddress: string;
+  clientGstin: string;
+  poNumber: string;
+  placeOfSupply: string;
+  discount: number;
+  taxRate: number;
+  reverseCharge: boolean;
+  signaturePath: string | null;
+  notes: string;
+  items: InvoiceItem[];
+}
+
+/**
+ * The tax treatment Rust derives from the supplier state and the place of supply
+ * (src-tauri/src/gst.rs); the form shows and sends the same value.
+ */
+export function deriveTaxType(settings: Settings, placeOfSupply: string): TaxType {
+  if (settings.gstRegistration === "unregistered") return "none";
+  return !placeOfSupply || placeOfSupply === settings.vendorStateCode ? "cgst_sgst" : "igst";
+}
+
+/** Mirrors vendor_snapshot() in src-tauri/src/commands/invoice.rs. */
+export function vendorSnapshotJson(settings: Settings): string {
+  return JSON.stringify({
+    vendorName: settings.vendorName,
+    vendorAddress: settings.vendorAddress,
+    vendorEmail: settings.vendorEmail,
+    vendorPhone: settings.vendorPhone,
+    vendorPan: settings.vendorPan,
+    vendorGstin: settings.gstRegistration === "unregistered" ? "" : settings.vendorGstin.trim(),
+    vendorStateCode: settings.vendorStateCode,
+    logoPath: settings.logoPath,
+    signaturePath: settings.signaturePath,
+    gstRegistration: settings.gstRegistration,
+    signatoryName: settings.signatoryName,
+    signatoryDesignation: settings.signatoryDesignation,
+  });
+}
+
+/**
+ * A draft-status Invoice built from the live form, so the real renderer can draw it before anything
+ * is issued. Totals use the same paise rounding Rust stores. `number` is the next number (a peek),
+ * not yet consumed.
+ */
+export function formToPreviewInvoice(form: PreviewForm, number: string, settings: Settings, bank: BankAccount | null): Invoice {
+  const taxType = deriveTaxType(settings, form.placeOfSupply);
+  const registered = settings.gstRegistration !== "unregistered";
+  const totals = computeTotals(form.items, form.discount, taxType, form.taxRate);
+  const items = form.items.map((item, i) => ({ ...item, amount: computeLineAmount(item), sortOrder: i }));
+  return {
+    id: 0,
+    number,
+    seriesId: null,
+    invoiceDate: form.invoiceDate,
+    dueDate: form.dueDate,
+    clientId: form.clientId,
+    clientName: form.clientName,
+    clientAddress: form.clientAddress,
+    clientGstin: form.clientGstin,
+    poNumber: form.poNumber,
+    placeOfSupplyStateCode: form.placeOfSupply,
+    subtotal: totals.subtotal,
+    discount: totals.discount,
+    taxType,
+    taxRate: form.taxRate,
+    cgstAmount: totals.cgst,
+    sgstAmount: totals.sgst,
+    igstAmount: totals.igst,
+    total: totals.total,
+    amountInWords: amountInWords(totals.total),
+    bankAccountId: bank?.id ?? null,
+    bankSnapshot: bank ? JSON.stringify(bank) : "",
+    signaturePath: form.signaturePath,
+    notes: form.notes,
+    status: "draft",
+    createdAt: "",
+    updatedAt: "",
+    docType: registered ? "tax_invoice" : "invoice",
+    reverseCharge: registered && form.reverseCharge,
+    vendorSnapshot: vendorSnapshotJson(settings),
+    snapshotOrigin: "issued",
+    cancelledAt: null,
+    cancelReason: "",
+    archivedPdfSha256: null,
+    renderPrefs: "",
+    items,
+  };
+}