Add payments with TDS, invoice detail view and filterable History (Phase E2)

- Migration M8 adds payments (cash, TDS, mode, reference). Payment status is derived, never stored: unpaid, partially paid, paid, or overdue. Overpayment is rejected, TDS counts toward settling, and only issued invoices accept payments. list_invoices and get_invoice carry the paid, TDS and balance summary.
- New invoice detail view: archived-PDF preview, payments list, Record payment modal, Export, Duplicate (as a draft), Re-render and Cancel. An invoice with payments cannot be cancelled until they are deleted.
- History gets search, status, financial-year, month and client filters, sortable columns, pagination, footer totals and an FY summary. Cancelled invoices are excluded from totals. The per-row archive-status calls are gone; archiving is on the detail page.
- Rust tests for payments and the list summary; vitest for fiscal-year, money, filter and duplicate helpers.

PDF output and fingerprint goldens are unchanged. The new UI has not been run in a webview yet.

src/hooks/useReturnFocus.ts

@@ -0,0 +1,21 @@
+import { useEffect, useRef } from "react";
+
+/**
+ * Gives focus back to whatever had it when a modal opened, once the modal closes. Carbon's Modal only
+ * does this with a launcher ref, which does not suit modals opened from table rows or menus.
+ */
+export function useReturnFocus(open: boolean): void {
+  const origin = useRef<HTMLElement | null>(null);
+  const wasOpen = useRef(false);
+  useEffect(() => {
+    if (open && !wasOpen.current) {
+      origin.current = document.activeElement instanceof HTMLElement ? document.activeElement : null;
+    } else if (!open && wasOpen.current) {
+      const el = origin.current;
+      origin.current = null;
+      // After the modal has unmounted its focus trap.
+      if (el && el.isConnected) setTimeout(() => el.focus(), 0);
+    }
+    wasOpen.current = open;
+  }, [open]);
+}