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/types.ts

@@ -58,16 +58,80 @@ export interface InvoiceSeries {
   nextInvoiceNumber: string;
 }
 
+export type GstCategory = "registered_regular" | "unregistered" | "composition" | "sez" | "overseas";
+
+/** Informational only; the tax heads still derive from supplier state and place of supply. */
+export const GST_CATEGORY_LABELS: Record<GstCategory, string> = {
+  registered_regular: "Registered (regular)",
+  unregistered: "Unregistered",
+  composition: "Composition",
+  sez: "SEZ",
+  overseas: "Overseas",
+};
+
 export interface Client {
   id: number | null;
   name: string;
+  /** The composed multi-line address; what invoices store and print. */
   address: string;
   gstin: string;
+  /** Doubles as the address state and the default place of supply. */
   stateCode: string;
   poNumber: string;
   createdAt: string;
+  addressLine1: string;
+  addressLine2: string;
+  city: string;
+  pincode: string;
+  gstCategory: GstCategory;
+  /** Notes / payment terms text copied onto a new invoice when the client is picked. */
+  defaultNotes: string;
+  /** Null means the settings default. */
+  paymentTermsDays: number | null;
+  /** Read-only: how many invoices reference this client. */
+  invoiceCount?: number;
 }
 
+export const blankClient = (): Client => ({
+  id: null,
+  name: "",
+  address: "",
+  gstin: "",
+  stateCode: "",
+  poNumber: "",
+  createdAt: "",
+  addressLine1: "",
+  addressLine2: "",
+  city: "",
+  pincode: "",
+  gstCategory: "unregistered",
+  defaultNotes: "",
+  paymentTermsDays: null,
+});
+
+/** A saved line item. `ratePaise` is the amount for "fixed" and the per-unit rate for "rate". */
+export interface ItemPreset {
+  id: number | null;
+  description: string;
+  hsnSac: string;
+  mode: LineMode;
+  unit: LineUnit;
+  ratePaise: number;
+  sortOrder: number;
+  createdAt: string;
+}
+
+export const blankPreset = (): ItemPreset => ({
+  id: null,
+  description: "",
+  hsnSac: "",
+  mode: "fixed",
+  unit: "unit",
+  ratePaise: 0,
+  sortOrder: 0,
+  createdAt: "",
+});
+
 export interface InvoiceItem {
   id?: number | null;
   description: string;