v0.5.2 — desktop entry + hicolor icon

Added

• Desktop integration: share/applications/niri-screensaver.desktop
  and share/icons/hicolor/scalable/apps/niri-screensaver.svg. The
  .desktop entry exposes the primary niri-screensaver-ctl launch
  action plus two Actions= entries (Preview an effect inline,
  Enable/disable screensaver) so launchers like Fuzzel/Anyrun/Walker
  can right-click to the secondary actions. Icon is a charcoal-stroke
  variant of the existing plugin SVG so it stays visible across
  light and dark GTK/Qt themes (the plugin's stroke="white"
  version is intentional for Noctalia tinting and stays as-is). Both
  install.sh and the AUR PKGBUILDs ship them to their respective
  XDG paths and refresh gtk-update-icon-cache /
  update-desktop-database post-install (silent no-op if those
  tools aren't installed).
• AUR PKGBUILDs scaffolded under packaging/aur/ — both
  niri-screensaver (stable, tracks tags; pkgver=0.5.1 with the
  v0.5.1 tarball sha256 baked in) and niri-screensaver-git (HEAD,
  with a pkgver() that derives from git describe). Each ships a
  $pkgname.install post-install hook that points the user at the
  niri window-rule snippet, the Noctalia plugin symlink command, and
  the inline noctalia-customCommand.json fallback.
  Pre-generated .SRCINFO files included so AUR sync is a clean copy.
  packaging/aur/.gitignore excludes makepkg build artifacts (pkg/,
  src/, *.pkg.tar.*, fetched tarballs, etc.) so a stray
  git add -A from the repo root doesn't accidentally commit them.
  packaging/aur/README.md documents first-time publish and per-release
  update flow, including the structurally identical .gitignore that
  belongs in each AUR-side repo. All declared deps (alacritty,
  niri, jq, python-terminaltexteffects) plus optdeps
  (noctalia-shell, playerctl, wlrctl, ydotool, figlet)
  verified present in Arch repos or AUR. End-to-end test build
  confirmed: makepkg --verifysource passes, makepkg -d produces a
  92K niri-screensaver-0.5.1-1-any.pkg.tar.zst with the expected
  41-file layout, and the -git PKGBUILD's pkgver() resolves
  correctly to <tag>.r<count>.g<short-hash>.
• CI: .github/workflows/release.yml auto-creates a GitHub Release
  on any v* tag push. Pulls the matching ## [X.Y.Z] section out of
  CHANGELOG.md via awk and feeds it to gh release create --notes-file. Falls back to the bare tag name if no matching section
  exists. Backfilled releases for v0.4.0/v0.5.0/v0.5.1 so the
  "Releases" sidebar reflects the actual ship history. Future cuts
  only require git tag vX.Y.Z && git push origin vX.Y.Z.