Fix Linux installer ownership and path issues

- Use SCRIPT_DIR to find source files from any directory
- Create venv as target user (sudo -u) for correct ownership
- Add python3 existence check
- Add source file existence check
- Add group to systemd service
- Fix uninstall.sh root check

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

uninstall.sh

@@ -10,9 +10,9 @@ echo "===================================="
 echo "  Kao Uninstaller"
 echo "===================================="
 
-# Check permissions
-if [ ! -w "/opt" ]; then
-    echo "Error: Need write access"
+# Check if running as root
+if [ "$EUID" -ne 0 ]; then
+    echo "Error: Must run as root"
     echo "Run with: sudo $0"
     exit 1
 fi