Add the ERPNext settings UI, History and detail controls, and voiced.invoice.v1 export (Phase F3)

- Settings gets an Integrations tab: connection fields with a write-only API secret, a CA PEM option, Test connection with versions and warnings, live mapping lists loaded after a test, naming mode, toggles and its own Save.
- Pushing for a GST-registered vendor is refused until a connection test result is stored. The UI shows the reason and disables Send.
- History shows an ERPNext status column from one bulk call, a row Send and Open in ERPNext, and a bulk Send selected with a draft-or-submit confirm step and per-row results that never abort the batch. The detail view gets an ERPNext panel and per-payment send. All of it is hidden or disabled when the integration is not configured.
- Optional auto-push after issue runs in the background and never blocks or fails the issue.
- Generic export: a deterministic voiced.invoice.v1 JSON (and CSV line items) for selected invoices, documented in docs/voiced-invoice-v1.md with a golden test. Money is integer paise plus a decimal string.

The new screens have not been run in a webview yet. The ERPNext Payment Entry deduction fields still need a live check.

src/hooks/useGenericExport.ts

@@ -0,0 +1,39 @@
+import { useCallback, useState } from "react";
+import { save } from "@tauri-apps/plugin-dialog";
+import { api } from "../lib/api";
+import { exportInvoicesV1, type InvoiceExportFormat } from "../lib/invoiceExportV1";
+import { useToast } from "../components/ToastProvider";
+
+/** "Export as voiced.invoice.v1": JSON or line-item CSV of the given invoices, through the save dialog. */
+export function useGenericExport(): { busy: boolean; run: (ids: number[], format: InvoiceExportFormat) => Promise<void> } {
+  const toast = useToast();
+  const [busy, setBusy] = useState(false);
+  const run = useCallback(
+    async (ids: number[], format: InvoiceExportFormat) => {
+      setBusy(true);
+      try {
+        const done = await exportInvoicesV1(
+          {
+            getInvoice: api.getInvoice,
+            listPayments: api.listPayments,
+            listClients: api.listClients,
+            getLastExportDir: api.getLastExportDir,
+            save: ({ defaultPath, format: fmt }) =>
+              save({ defaultPath, filters: [fmt === "csv" ? { name: "CSV (line items)", extensions: ["csv"] } : { name: "JSON (voiced.invoice.v1)", extensions: ["json"] }] }),
+            writeExportFile: api.writeExportFile,
+            now: () => new Date(),
+          },
+          ids,
+          format,
+        );
+        if (done) toast.success(`Exported ${done.count} ${done.count === 1 ? "invoice" : "invoices"}`, done.path);
+      } catch (e) {
+        toast.error("Could not export the invoices", String(e));
+      } finally {
+        setBusy(false);
+      }
+    },
+    [toast],
+  );
+  return { busy, run };
+}