feat: initial noctalia-greeter release

QuickShell QML login greeter for greetd, styled after noctalia-shell
lockscreen. Runs under niri compositor as _greeter user. Theme is
live-synced from noctalia config via runit service.

- shell.qml: QuickShell greeter UI + greetd auth flow
- sync/: inotifywait theme sync daemon + runit service
- greetd/: niri compositor config + wrapper scripts
- install.sh: deployment helper
- test/check-syntax.sh: 19 syntax/structural checks (0 failures)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sync/noctalia-greeter-sync

@@ -0,0 +1,49 @@
+#!/bin/sh
+# Syncs noctalia theme to /var/lib/noctalia-greeter-theme/ for the _greeter user.
+export HOME=/home/moze
+export USER=moze
+
+THEME_DIR="/var/lib/noctalia-greeter-theme"
+COLORS="$HOME/.config/noctalia/colors.json"
+WALLPAPERS_JSON="$HOME/.cache/noctalia/wallpapers.json"
+
+do_sync() {
+    if [ -f "$COLORS" ]; then
+        cp -f "$COLORS" "$THEME_DIR/colors.json" 2>/dev/null
+        chmod 644 "$THEME_DIR/colors.json" 2>/dev/null
+    fi
+
+    if [ -f "$WALLPAPERS_JSON" ]; then
+        WP=$(python3 -c "
+import json, sys
+try:
+    d = json.load(open('$WALLPAPERS_JSON'))
+    m = d.get('wallpapers', {})
+    for s in ['eDP-1', 'DP-1'] + list(m.keys()):
+        if s in m:
+            p = m[s].get('dark') or m[s].get('light') or ''
+            if p:
+                print(p)
+                sys.exit(0)
+    fb = d.get('defaultWallpaper', '')
+    if fb: print(fb)
+except Exception as e:
+    sys.stderr.write(str(e) + chr(10))
+" 2>/dev/null)
+        if [ -n "$WP" ] && [ -f "$WP" ]; then
+            cp -f "$WP" "$THEME_DIR/wallpaper.jpg" 2>/dev/null
+            chmod 644 "$THEME_DIR/wallpaper.jpg" 2>/dev/null
+        fi
+    fi
+}
+
+do_sync
+
+exec inotifywait -m -q \
+    -e close_write,moved_to,create \
+    "$HOME/.config/noctalia/" \
+    "$HOME/.cache/noctalia/" 2>/dev/null |
+while IFS= read -r _line; do
+    sleep 0.3
+    do_sync
+done

sync/run

@@ -0,0 +1,3 @@
+#!/bin/sh
+exec 2>&1
+exec chpst -u moze env HOME=/home/moze USER=moze /usr/local/bin/noctalia-greeter-sync