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.

app/build.gradle.kts

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 import java.util.Properties
 
 plugins {

app/src/androidTest/kotlin/net/jeena/pacer/PacerScreenTest.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import androidx.compose.ui.test.assertIsDisplayed

app/src/main/kotlin/net/jeena/pacer/AudioEngine.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import android.media.AudioAttributes

app/src/main/kotlin/net/jeena/pacer/BeepGenerator.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import kotlin.math.sin

app/src/main/kotlin/net/jeena/pacer/BpmCalculator.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 object BpmCalculator {

app/src/main/kotlin/net/jeena/pacer/MainActivity.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import android.content.Intent

app/src/main/kotlin/net/jeena/pacer/PacerService.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import android.app.NotificationChannel

app/src/main/kotlin/net/jeena/pacer/PacerViewModel.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import android.app.Application

app/src/main/kotlin/net/jeena/pacer/SettingsRepository.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import android.content.Context

app/src/main/kotlin/net/jeena/pacer/ui/PacerScreen.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer.ui
 
 import android.content.res.Configuration

app/src/main/kotlin/net/jeena/pacer/ui/theme/Color.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer.ui.theme
 
 import androidx.compose.ui.graphics.Color

app/src/main/kotlin/net/jeena/pacer/ui/theme/Font.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer.ui.theme
 
 import androidx.compose.ui.text.font.Font

app/src/main/kotlin/net/jeena/pacer/ui/theme/Theme.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer.ui.theme
 
 import androidx.compose.material3.MaterialTheme

app/src/main/kotlin/net/jeena/pacer/ui/theme/Type.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer.ui.theme
 
 import androidx.compose.material3.Typography

app/src/test/kotlin/net/jeena/pacer/BeepGeneratorTest.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import org.junit.Assert.assertEquals

app/src/test/kotlin/net/jeena/pacer/BpmCalculatorTest.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import org.junit.Assert.assertEquals

app/src/test/kotlin/net/jeena/pacer/SettingsRepositoryTest.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Jeena <hello@jeena.net>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package net.jeena.pacer
 
 import android.content.Context