Add app icon, desktop entry, and release packaging script

- Generate app icon (teal droplet + music note with sound waves)
- Create icon sizes: 16, 32, 48, 64, 128, 256, 512, 1024px
- Add .desktop file for Linux desktop integration
- Set GTK window icon from bundled 256px icon
- Bundle icon into release build via CMakeLists.txt
- Add scripts/package-release.sh that builds, bundles, and creates
  a distributable tarball with install/uninstall scripts
- No Flutter SDK needed to run: the bundle is fully standalone
  (users only need GTK3 and mpv system libraries)

Author: oliveiraethales

AGENTS.md

@@ -95,7 +95,7 @@ sudo pacman -S clang ninja cmake gtk3 pkg-config mpv
 - [ ] **Shuffle & repeat** — toggle buttons in now playing bar
 - [ ] **Keyboard shortcuts** — Space=play/pause, Left/Right=seek, Ctrl+F=search
 - [ ] **MPRIS integration** — system media key support (play/pause/next/prev from DE)
-- [ ] **App icon** — custom icon for desktop shortcut and window
+- [x] **App icon** — custom icon for desktop shortcut and window
 - [ ] **Gapless playback polish** — pre-fetch next track manifest for seamless transition
 - [ ] **Offline caching** — cache album art and metadata locally
 - [ ] **Settings screen** — audio quality selector, audio output device, cache management

assets/icon/torrential-128.png

@@ -126,3 +126,9 @@ if(NOT CMAKE_BUILD_TYPE MATCHES "Debug")
   install(FILES "${AOT_LIBRARY}" DESTINATION "${INSTALL_BUNDLE_LIB_DIR}"
     COMPONENT Runtime)
 endif()
+
+# Install application icon.
+set(INSTALL_BUNDLE_ICON_DIR "${CMAKE_INSTALL_PREFIX}/data/icon")
+install(FILES "${CMAKE_CURRENT_SOURCE_DIR}/../assets/icon/torrential-256.png"
+  DESTINATION "${INSTALL_BUNDLE_ICON_DIR}"
+  COMPONENT Runtime)