Add security hardening, config system, and integration tests

- Password wiping after PAM auth (bytearray zeroed)
- TOML config loading (/etc/moonlock/ and ~/.config/moonlock/)
- Config controls fingerprint_enabled and background_path
- Integration tests for password/fingerprint auth flows
- Security tests for bypass prevention and data cleanup
- 51 tests passing

config/moonlock.toml.example

@@ -0,0 +1,8 @@
+# Moonlock configuration
+# Copy to /etc/moonlock/moonlock.toml or ~/.config/moonlock/moonlock.toml
+
+# Path to background wallpaper image (optional)
+# background_path = "/usr/share/wallpapers/moon.jpg"
+
+# Enable fingerprint authentication via fprintd (default: true)
+# fingerprint_enabled = true