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/Cargo.toml

@@ -6,6 +6,9 @@ exclude = ["fuzz"]
 [workspace.package]
 version = "3.0.2"
 edition = "2024"
+license = "AGPL-3.0-only"
+authors = ["Andy Dixon <andy@dixon.cx>"]
+repository = "https://github.com/andydixon/nexus"
 
 [profile.dev.package."*"]
 opt-level = 2