Adopt GPL-3.0-or-later license

TutaBridge links Tuta's Rust SDK, which is part of the GPLv3-licensed
tutanota project, so the bridge must carry the same license. Add the
full GPLv3 text, set `license = "GPL-3.0-or-later"` on all three
crates, and note it in the README.

src-tauri/Cargo.toml

@@ -3,6 +3,7 @@ name = "tutabridge-gui"
 version = "0.1.0"
 edition = "2021"
 rust-version = "1.84.0"
+license = "GPL-3.0-or-later"
 
 [build-dependencies]
 tauri-build = { version = "2", features = [] }