Add toasts, Settings tabs, Clients page and item presets (Phase E1)

- Global Carbon toast stack replaces scattered inline notices; errors persist until closed.
- Settings is split into eight tabs with one Save and a remembered last tab; Numbering embeds the series settings.
- New Clients page with a DataTable, shared add/edit modal, structured addresses, GST category (informational), per-client notes and payment terms. The composed address string is kept for back-compat. Deleting a client warns about invoice count; issued invoices keep their own snapshot.
- Migration M7 adds the client columns and item_presets; existing clients with a GSTIN are backfilled to registered_regular.
- New invoice form: client ComboBox with type-ahead and inline create, client defaults applied on selection, and an Add from preset control.
- Rust tests for M7 (fresh and v6 upgrade), client validation and CRUD, presets CRUD; vitest for the toast store and new helpers.

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

src/components/ToastProvider.tsx

@@ -0,0 +1,63 @@
+import { createContext, useContext, useMemo, useRef, useSyncExternalStore, type ReactNode } from "react";
+import { ToastNotification } from "@carbon/react";
+import { createToastStore, type ToastStore } from "../lib/toastStore";
+
+export interface ToastApi {
+  success: (title: string, subtitle?: string) => number;
+  info: (title: string, subtitle?: string) => number;
+  warning: (title: string, subtitle?: string) => number;
+  /** Errors stay until the user closes them. */
+  error: (title: string, subtitle?: string) => number;
+  dismiss: (id: number) => void;
+}
+
+const ToastContext = createContext<ToastApi | null>(null);
+
+function apiFor(store: ToastStore): ToastApi {
+  return {
+    success: (t, s) => store.add("success", t, s),
+    info: (t, s) => store.add("info", t, s),
+    warning: (t, s) => store.add("warning", t, s),
+    error: (t, s) => store.add("error", t, s),
+    dismiss: store.dismiss,
+  };
+}
+
+/** Stacked top-right toasts. Place it inside the Carbon <Theme> so the tokens apply. */
+export function ToastProvider({ children }: { children: ReactNode }) {
+  const storeRef = useRef<ToastStore | null>(null);
+  if (!storeRef.current) storeRef.current = createToastStore();
+  const store = storeRef.current;
+  const toasts = useSyncExternalStore(store.subscribe, store.getSnapshot);
+  const api = useMemo(() => apiFor(store), [store]);
+
+  return (
+    <ToastContext.Provider value={api}>
+      {children}
+      {/* The live region stays mounted so screen readers announce toasts as they are added. */}
+      <div className="voiced-toasts" aria-live="polite" aria-relevant="additions" role="region" aria-label="Notifications">
+        {toasts.map((t) => (
+          <ToastNotification
+            key={t.id}
+            kind={t.kind}
+            lowContrast
+            title={t.title}
+            subtitle={t.subtitle}
+            role={t.kind === "error" ? "alert" : "status"}
+            onClose={() => {
+              store.dismiss(t.id);
+              return false;
+            }}
+            onCloseButtonClick={() => store.dismiss(t.id)}
+          />
+        ))}
+      </div>
+    </ToastContext.Provider>
+  );
+}
+
+export function useToast(): ToastApi {
+  const api = useContext(ToastContext);
+  if (!api) throw new Error("useToast must be used inside <ToastProvider>");
+  return api;
+}