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/Cargo.toml

@@ -2,6 +2,9 @@
 name = "nexus"
 version.workspace = true
 edition.workspace = true
+license.workspace = true
+authors.workspace = true
+repository.workspace = true
 build = "build.rs"
 
 [[bin]]