license: release Nexus Terminal under the GNU AGPL v3

- LICENSE: the GNU Affero General Public License, version 3 (verbatim
  from gnu.org).
- SPDX-License-Identifier: AGPL-3.0-only plus a copyright line on every
  first-party source file (Rust, shell, Python, PHP); vendored crates keep
  their own licenses.
- Cargo manifests carry license/authors/repository; the macOS bundle
  copyright and the About page state the license and link the source.
- Release zips (and the macOS app bundle) include LICENSE.
- README: License section.

rust/app/src/sidebar.rs

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+// Copyright (C) 2026 Andy Dixon
+
 //! The connection sidebar: Local Machine, Favourites, Recent, folders, Ungrouped; fuzzy search;
 //! click to connect, Ctrl/Cmd-click to multi-select, double-click to edit; bulk actions.