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/api.ts

@@ -2,6 +2,7 @@ import { invoke } from "@tauri-apps/api/core";
 import type {
   BankAccount,
   Client,
+  DraftSummary,
   Invoice,
   InvoiceInput,
   InvoiceSeries,
@@ -27,10 +28,18 @@ export const api = {
   deleteClient: (id: number) => invoke<void>("delete_client", { id }),
 
   peekNextInvoiceNumber: () => invoke<string>("peek_next_invoice_number"),
-  createInvoice: (input: InvoiceInput) => invoke<Invoice>("create_invoice", { input }),
+  issueInvoice: (input: InvoiceInput, renderPrefs: Record<string, unknown>) =>
+    invoke<Invoice>("issue_invoice", { input, renderPrefs }),
+  cancelInvoice: (id: number, reason: string) =>
+    invoke<Invoice>("cancel_invoice", { id, reason }),
   listInvoices: () => invoke<InvoiceSummary[]>("list_invoices"),
   getInvoice: (id: number) => invoke<Invoice>("get_invoice", { id }),
-  deleteInvoice: (id: number) => invoke<void>("delete_invoice", { id }),
+
+  saveDraft: (id: number | null, payload: unknown) =>
+    invoke<number>("save_draft", { id, payload }),
+  listDrafts: () => invoke<DraftSummary[]>("list_drafts"),
+  getDraft: (id: number) => invoke<unknown>("get_draft", { id }),
+  deleteDraft: (id: number) => invoke<void>("delete_draft", { id }),
 
   importAsset: (sourcePath: string, kind: string) =>
     invoke<string>("import_asset", { sourcePath, kind }),