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/App.tsx

@@ -6,10 +6,20 @@ import { AppShell, type AppView } from "./components/AppShell";
 import Onboarding from "./views/Onboarding";
 import NewInvoice from "./views/NewInvoice";
 import InvoiceHistory from "./views/InvoiceHistory";
-import SeriesSettings from "./views/SeriesSettings";
+import Clients from "./views/Clients";
+import { ToastProvider, useToast } from "./components/ToastProvider";
 import AppSettings from "./views/AppSettings";
 
 export default function App() {
+  return (
+    <ToastProvider>
+      <AppInner />
+    </ToastProvider>
+  );
+}
+
+function AppInner() {
+  const toast = useToast();
   const [settings, setSettings] = useState<Settings | null>(null);
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
@@ -81,7 +91,7 @@ export default function App() {
     try {
       await api.saveSettings(next);
     } catch (e) {
-      setError(String(e));
+      toast.error("Could not save the theme", String(e));
     }
   };
 
@@ -98,8 +108,8 @@ export default function App() {
         <div hidden={view !== "invoices"}>
           <InvoiceHistory settings={settings} active={view === "invoices"} />
         </div>
-        <div hidden={view !== "series"}>
-          <SeriesSettings active={view === "series"} />
+        <div hidden={view !== "clients"}>
+          <Clients active={view === "clients"} />
         </div>
         {view === "settings" && <AppSettings settings={settings} onSaved={refreshSettings} />}
       </AppShell>