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/validators.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { gstinChecksumError, gstinFullError, vendorGstinError } from "./validators";
+import { clientGstinStateError, gstinChecksumError, gstinFullError, vendorGstinError } from "./validators";
 
 // Same GSTINs as the Rust tests in src-tauri/src/gst.rs.
 describe("GSTIN checksum", () => {
@@ -25,3 +25,19 @@ describe("GSTIN checksum", () => {
     expect(vendorGstinError("regular", "27AAPFU0939F1ZV", "AAPFU0939F", "27")).toBeUndefined();
   });
 });
+
+describe("clientGstinStateError", () => {
+  it("allows empty and NA", () => {
+    expect(clientGstinStateError("", "29")).toBeUndefined();
+    expect(clientGstinStateError("NA", "29")).toBeUndefined();
+  });
+  it("checks shape and checksum", () => {
+    expect(clientGstinStateError("29ABCDE1234F1Z5", "29")).toMatch(/check digit/);
+    expect(clientGstinStateError("29ABC", "")).toMatch(/15-character/);
+  });
+  it("checks the state prefix only when a state is chosen", () => {
+    expect(clientGstinStateError("29ABCDE1234F1ZW", "29")).toBeUndefined();
+    expect(clientGstinStateError("29abcde1234f1zw", "")).toBeUndefined();
+    expect(clientGstinStateError("29ABCDE1234F1ZW", "27")).toMatch(/starts with 29/);
+  });
+});