Add PKGBUILD and remove stale moongreet icon

Add Arch Linux PKGBUILD for makepkg installation. Installs the
Python package, PAM config, and example config file. Fix pkgver()
to handle repos without tags (sed always exits 0 on empty input).

Remove leftover moongreet icon that was accidentally included in
the data directory.

pkg/PKGBUILD

@@ -0,0 +1,57 @@
+# ABOUTME: PKGBUILD for Moonlock — secure Wayland lockscreen.
+# ABOUTME: Builds from git source with automatic version detection.
+
+# Maintainer: Dominik Kressler
+
+pkgname=moonlock-git
+pkgver=0.1.0
+pkgrel=1
+pkgdesc="A secure Wayland lockscreen with GTK4, PAM and fingerprint support"
+arch=('any')
+url="https://gitea.moonarch.de/nevaforget/moonlock"
+license=('MIT')
+depends=(
+    'python'
+    'python-gobject'
+    'gtk4'
+    'gtk4-layer-shell'
+    'pam'
+)
+makedepends=(
+    'git'
+    'python-build'
+    'python-installer'
+    'python-hatchling'
+)
+optdepends=(
+    'fprintd: fingerprint authentication support'
+)
+provides=('moonlock')
+conflicts=('moonlock')
+source=("git+${url}.git")
+sha256sums=('SKIP')
+
+pkgver() {
+    cd "$srcdir/moonlock"
+    if git describe --long --tags 2>/dev/null; then
+        git describe --long --tags | sed 's/^v//;s/-/.r/;s/-/./'
+    else
+        printf "0.0.0.r%s.%s" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
+    fi
+}
+
+build() {
+    cd "$srcdir/moonlock"
+    python -m build --wheel --no-isolation
+}
+
+package() {
+    cd "$srcdir/moonlock"
+    python -m installer --destdir="$pkgdir" dist/*.whl
+
+    # PAM configuration
+    install -Dm644 config/moonlock-pam "$pkgdir/etc/pam.d/moonlock"
+
+    # Example config
+    install -Dm644 config/moonlock.toml.example "$pkgdir/etc/moonlock/moonlock.toml.example"
+}