feat(packaging): bundle app icon + Linux desktop entry; wire into AUR package

- release tarballs now ship podtui.png (assets/App Icon/App Icon.png, 512px)
  on every platform; Linux tarballs additionally carry podtui.desktop
  (Terminal=true so launchers open the TUI in a terminal)
- AUR PKGBUILD installs both: icon to hicolor 512x512, entry to applications/
- build.ts bundles the icon/desktop into dist before tarballing

packaging/podtui.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=PodTui
+GenericName=Podcast Player
+Comment=Terminal podcast and audio player with waveform visualization
+Exec=podtui
+Icon=podtui
+Terminal=true
+Categories=Audio;AudioVideo;Player;
+Keywords=podcast;audio;player;terminal;
+StartupNotify=false