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/AppShell.tsx

@@ -11,12 +11,12 @@ import {
 import { Asleep, Light } from "@carbon/icons-react";
 import type { ReactNode } from "react";
 
-export type AppView = "new" | "invoices" | "series" | "settings";
+export type AppView = "new" | "invoices" | "clients" | "settings";
 
 const NAV: { id: AppView; label: string }[] = [
   { id: "new", label: "New invoice" },
   { id: "invoices", label: "Invoices" },
-  { id: "series", label: "Series" },
+  { id: "clients", label: "Clients" },
   { id: "settings", label: "Settings" },
 ];