fix(desktop): improve Linux desktop icon integration (#334)

Refs #330

## Summary
- add standard Linux hicolor icon sizes to the Tauri package outputs
- enable the GTK app id on Linux and ship a matching reverse-DNS desktop
entry alias for shell association
- mark the alias desktop entry `NoDisplay=true` so it does not surface
as a duplicate launcher in desktop menus
- include the same alias desktop entry for AppImage so the fix is not
limited to deb/rpm packages

## Validation
- confirmed in the Linux VM that the desktop-integrated launch no longer
shows the generic taskbar icon
- verified the alias desktop entry is now hidden from app menus via
`NoDisplay=true`
- attempted a fresh `tauri build --bundles deb`; the build still hits
the known optional `@tauri-apps/cli` native-binding issue in this
workspace after prebuild, not a code/config error from this PR

packages/tauri-app/src-tauri/icons/linux/ai.neuralnomads.codenomad.client.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Categories=
+Exec=codenomad-tauri
+StartupWMClass=codenomad-tauri
+Icon=codenomad-tauri
+Name=CodeNomad
+NoDisplay=true
+Terminal=false
+Type=Application