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/styles/carbon.scss

@@ -148,6 +148,23 @@ body {
   display: none !important;
 }
 
+.voiced-toasts {
+  position: fixed;
+  top: 3.5rem; /* below the 3rem Carbon header */
+  right: 1rem;
+  z-index: 9100;
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+  max-width: calc(100vw - 2rem);
+  pointer-events: none;
+}
+
+.voiced-toasts .cds--toast-notification {
+  pointer-events: auto;
+  margin: 0;
+}
+
 .voiced-page {
   padding: 2rem;
   max-width: 1600px;