chore: License the project under GPL-3.0-or-later

In preparation for publishing the source publicly: without a license
file the code would default to all-rights-reserved, so nobody could
legally use or modify it. Add the GPL v3 text, a license section in
the README, and REUSE-style SPDX headers in every Kotlin source and
Gradle build script so per-file licensing is machine-readable.

The bundled JetBrains Mono font stays under its own SIL Open Font
License 1.1, whose text must accompany redistribution. OFL.txt lives
at the repo root rather than next to the fonts because aapt rejects
non-font files in res/font.

build.gradle.kts

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 plugins {
     alias(libs.plugins.android.application) apply false
     alias(libs.plugins.kotlin.android) apply false