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.ts

@@ -0,0 +1,38 @@
+const KEY = "voiced.settingsTab";
+
+export const SETTINGS_TABS = [
+  { id: "business", label: "Business" },
+  { id: "branding", label: "Branding" },
+  { id: "bank", label: "Bank & payments" },
+  { id: "defaults", label: "Invoice defaults" },
+  { id: "numbering", label: "Numbering" },
+  { id: "templates", label: "Templates & export" },
+  { id: "fonts", label: "Fonts" },
+  { id: "data", label: "Data" },
+] as const;
+
+export type SettingsTabId = (typeof SETTINGS_TABS)[number]["id"];
+
+/** Index of a stored tab id; 0 (Business) for anything unknown. */
+export function parseSettingsTab(value: string | null | undefined): number {
+  const index = SETTINGS_TABS.findIndex((t) => t.id === value);
+  return index < 0 ? 0 : index;
+}
+
+/** The last Settings tab this viewer used. A per-viewer convenience only. */
+export function loadSettingsTab(): number {
+  try {
+    return parseSettingsTab(localStorage.getItem(KEY));
+  } catch {
+    return 0;
+  }
+}
+
+export function saveSettingsTab(index: number): void {
+  try {
+    const tab = SETTINGS_TABS[index];
+    if (tab) localStorage.setItem(KEY, tab.id);
+  } catch {
+    // Not remembering is harmless.
+  }
+}