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/commands/invoice.rs

@@ -338,15 +338,16 @@ pub fn issue_invoice_impl(
         Some(id) => Some(id),
         None if input.save_client && !input.client_name.trim().is_empty() => {
             tx.execute(
-                "INSERT INTO clients (name, address, gstin, state_code, po_number, created_at)
-                 VALUES (?1, ?2, ?3, ?4, ?5, ?6)",
+                "INSERT INTO clients (name, address, gstin, state_code, po_number, created_at, gst_category)
+                 VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7)",
                 params![
                     input.client_name,
                     input.client_address,
                     client_gstin,
                     stored_pos,
                     input.po_number,
-                    now
+                    now,
+                    if client_gstin.is_empty() { "unregistered" } else { "registered_regular" }
                 ],
             )
             .map_err(db)?;