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/toastStore.ts

@@ -0,0 +1,83 @@
+export type ToastKind = "success" | "info" | "warning" | "error";
+
+export interface Toast {
+  id: number;
+  kind: ToastKind;
+  title: string;
+  subtitle?: string;
+}
+
+/** Success, info and warning toasts leave by themselves; errors stay until closed. */
+export const AUTO_DISMISS_MS = 5000;
+/** Oldest toasts are dropped beyond this, so a failing loop cannot fill the screen. */
+export const MAX_TOASTS = 5;
+
+type TimerId = unknown;
+
+export interface ToastStoreOptions {
+  setTimer?: (fn: () => void, ms: number) => TimerId;
+  clearTimer?: (id: TimerId) => void;
+}
+
+export interface ToastStore {
+  add: (kind: ToastKind, title: string, subtitle?: string) => number;
+  dismiss: (id: number) => void;
+  clear: () => void;
+  subscribe: (listener: () => void) => () => void;
+  getSnapshot: () => readonly Toast[];
+}
+
+/** A tiny external store (useSyncExternalStore-compatible); the timers are injectable for tests. */
+export function createToastStore(options: ToastStoreOptions = {}): ToastStore {
+  const setTimer = options.setTimer ?? ((fn, ms) => setTimeout(fn, ms));
+  const clearTimer = options.clearTimer ?? ((id) => clearTimeout(id as ReturnType<typeof setTimeout>));
+  let toasts: readonly Toast[] = [];
+  let nextId = 1;
+  const timers = new Map<number, TimerId>();
+  const listeners = new Set<() => void>();
+
+  const emit = () => listeners.forEach((l) => l());
+
+  const drop = (id: number) => {
+    const timer = timers.get(id);
+    if (timer !== undefined) {
+      clearTimer(timer);
+      timers.delete(id);
+    }
+  };
+
+  const dismiss = (id: number) => {
+    drop(id);
+    if (!toasts.some((t) => t.id === id)) return;
+    toasts = toasts.filter((t) => t.id !== id);
+    emit();
+  };
+
+  return {
+    add(kind, title, subtitle) {
+      const id = nextId++;
+      toasts = [...toasts, { id, kind, title, subtitle }];
+      while (toasts.length > MAX_TOASTS) {
+        drop(toasts[0].id);
+        toasts = toasts.slice(1);
+      }
+      if (kind !== "error") timers.set(id, setTimer(() => dismiss(id), AUTO_DISMISS_MS));
+      emit();
+      return id;
+    },
+    dismiss,
+    clear() {
+      timers.forEach((t) => clearTimer(t));
+      timers.clear();
+      toasts = [];
+      emit();
+    },
+    subscribe(listener) {
+      listeners.add(listener);
+      return () => {
+        listeners.delete(listener);
+      };
+    },
+    getSnapshot: () => toasts,
+  };
+}