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-tauri/src/lib.rs

@@ -121,6 +121,9 @@ pub fn run() {
             commands::clients::list_clients,
             commands::clients::save_client,
             commands::clients::delete_client,
+            commands::presets::list_item_presets,
+            commands::presets::save_item_preset,
+            commands::presets::delete_item_preset,
             commands::invoice::peek_next_invoice_number,
             commands::invoice::issue_invoice,
             commands::invoice::cancel_invoice,