feat: add opt-in Discord Rich Presence on Android

Use the official Discord Social SDK through JNI to share built-in player activity, clear presence when playback stops, and guard against late callbacks after disabling. Package the SDK optionally through SPOTIFLAC_DISCORD_SDK_DIR with license notices.

Credit: @itsmegaaa for the Rich Presence initiative and contribution in #575 and #576.

Validation: 15 Dart tests, scoped Flutter analysis, and one Android 16 ARM64 SDK/JNI instrumentation test passed. Visible presence on a signed-in Discord profile remains unverified.

android/app/src/androidTest/kotlin/com/zarz/spotiflac/discord/DiscordPresenceTest.kt

@@ -0,0 +1,54 @@
+package com.zarz.spotiflac.discord
+
+import android.content.Intent
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import androidx.test.platform.app.InstrumentationRegistry
+import com.zarz.spotiflac.BuildConfig
+import com.zarz.spotiflac.MainActivity
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Assume.assumeTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+
+/** Exercises the actual JNI library/SDK in the emulator, without account credentials. */
+@RunWith(AndroidJUnit4::class)
+class DiscordPresenceTest {
+    @Test
+    fun testSdkLifecycleAndLateCallbacks() {
+        val instrumentation = InstrumentationRegistry.getInstrumentation()
+        assumeTrue("Build with SPOTIFLAC_DISCORD_SDK_DIR", BuildConfig.HAS_DISCORD_SDK)
+        assumeTrue("This test must not publish to a signed-in account",
+            instrumentation.targetContext.packageManager.getLaunchIntentForPackage("com.discord") == null)
+        val activity = instrumentation.startActivitySync(
+            Intent(instrumentation.targetContext, MainActivity::class.java)
+                .addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
+        )
+        try {
+            instrumentation.runOnMainSync {
+                assertEquals("discord_missing", DiscordPresenceBridge.configure(activity, true))
+                assertEquals("disabled", DiscordPresenceBridge.configure(activity, false))
+                System.loadLibrary("spotiflac_discord")
+                Class.forName("com.discord.socialsdk.DiscordSocialSdkInit")
+                    .getMethod("setEngineActivity", android.app.Activity::class.java)
+                    .invoke(null, activity)
+                DiscordNative.start(1549854098801692862L)
+                DiscordNative.update("Track 🎵", "Lead & Guest", "Album", "", 1000, 2000)
+                DiscordNative.clear()
+                DiscordNative.stop()
+            }
+            repeat(20) {
+                Thread.sleep(50)
+                instrumentation.runOnMainSync { assertEquals("disabled", DiscordNative.poll()) }
+            }
+            instrumentation.runOnMainSync {
+                DiscordNative.start(1549854098801692862L)
+                assertEquals("ready", DiscordNative.poll())
+                DiscordNative.stop()
+                assertEquals("disabled", DiscordNative.poll())
+            }
+        } finally {
+            instrumentation.runOnMainSync { activity.finish() }
+        }
+    }
+}