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/settingsTab.test.ts

@@ -0,0 +1,17 @@
+import { describe, expect, it } from "vitest";
+import { SETTINGS_TABS, loadSettingsTab, parseSettingsTab, saveSettingsTab } from "./settingsTab";
+
+describe("settings tab persistence", () => {
+  it("maps stored ids to indexes and falls back to the first tab", () => {
+    expect(parseSettingsTab("fonts")).toBe(SETTINGS_TABS.findIndex((t) => t.id === "fonts"));
+    expect(parseSettingsTab("nope")).toBe(0);
+    expect(parseSettingsTab(null)).toBe(0);
+  });
+
+  it("does not throw when storage is unavailable", () => {
+    // The node test environment has no localStorage at all.
+    expect(loadSettingsTab()).toBe(0);
+    expect(() => saveSettingsTab(2)).not.toThrow();
+    expect(() => saveSettingsTab(99)).not.toThrow();
+  });
+});