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/lib/api.ts

@@ -7,6 +7,7 @@ import type {
   InvoiceInput,
   InvoiceSeries,
   InvoiceSummary,
+  ItemPreset,
   Settings,
 } from "./types";
 import type { LogoAsset } from "./logo";
@@ -36,6 +37,10 @@ export const api = {
   saveClient: (client: Client) => invoke<Client>("save_client", { client }),
   deleteClient: (id: number) => invoke<void>("delete_client", { id }),
 
+  listItemPresets: () => invoke<ItemPreset[]>("list_item_presets"),
+  saveItemPreset: (preset: ItemPreset) => invoke<ItemPreset>("save_item_preset", { preset }),
+  deleteItemPreset: (id: number) => invoke<void>("delete_item_preset", { id }),
+
   peekNextInvoiceNumber: () => invoke<string>("peek_next_invoice_number"),
   issueInvoice: (input: InvoiceInput, renderPrefs: object) =>
     invoke<Invoice>("issue_invoice", { input, renderPrefs }),