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.

lib/l10n/arb/app_en.arb

@@ -1,4 +1,8 @@
 {
+  "discordRichPresence": "Discord Rich Presence",
+  "discordPresenceDescription": "Share the track playing in the built-in player on Discord. Requires Discord to be installed and signed in.",
+  "discordPresenceMissing": "Install Discord and sign in, then turn this setting off and on again.",
+  "discordPresenceUnavailable": "Discord Rich Presence is unavailable in this build or on this device.",
   "@@locale": "en",
   "@@last_modified": "2026-04-28",
   "appName": "SpotiFLAC Mobile",

lib/l10n/arb/app_id.arb

@@ -1,4 +1,8 @@
 {
+  "discordRichPresence": "Discord Rich Presence",
+  "discordPresenceDescription": "Bagikan lagu yang diputar di pemutar bawaan ke Discord. Discord harus terpasang dan sudah login.",
+  "discordPresenceMissing": "Pasang Discord dan login, lalu matikan dan nyalakan kembali pengaturan ini.",
+  "discordPresenceUnavailable": "Discord Rich Presence tidak tersedia pada build atau perangkat ini.",
   "@@locale": "id",
   "@@last_modified": "2026-04-28",
   "libraryEnableLocalLibrarySubtitle": "Scan and track your existing music",