chore(pkg): add Arch Linux PKGBUILDs for game client and sync server

- pkg/solitaire-quest/PKGBUILD: builds solitaire_app binary, depends on
  alsa-lib, libxkbcommon, systemd-libs (Bevy Linux requirements); check()
  runs only non-Bevy crates (solitaire_core, solitaire_sync) since Bevy
  integration tests require a GPU/display unavailable in chroot
- pkg/solitaire-quest-server/PKGBUILD: builds solitaire_server binary,
  installs systemd service unit and hardened environment file template
- pkg/solitaire-quest-server/solitaire-quest-server.service: systemd unit
  with ProtectSystem=strict, NoNewPrivileges, dedicated service user
- pkg/solitaire-quest-server/server.env: documented env template installed
  to /etc/solitaire-quest-server/server.env (mode 0640, listed in backup=)
- LICENSE: add MIT license
- Cargo.toml: add license = "MIT" to [workspace.package]
- All member crates: add license.workspace = true

Both PKGBUILDs follow the Arch Rust package guidelines:
  prepare() uses --locked + cargo fetch
  build() uses --frozen --release -p <crate>
  RUSTUP_TOOLCHAIN=stable and CARGO_TARGET_DIR=target set in each stage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_app/Cargo.toml

@@ -1,6 +1,7 @@
 [package]
 name    = "solitaire_app"
 version.workspace = true
+license.workspace = true
 edition.workspace = true
 
 [[bin]]