refactor: apply CLAUDE.md code quality improvements and add packaging

- Add #![forbid(unsafe_code)] to main.rs (issue #3)
- Replace raw ANSI escape codes with owo-colors crate (issue #2)
- Replace manual HOME path construction with dirs::home_dir() (issue #5)
- Ship umutray.service as a static file; service::install() substitutes
  the binary path at install time instead of generating the unit at runtime
- Add packaging/PKGBUILD following Arch Rust package guidelines
- Add CLAUDE.md tracking refactor tasks
- setup.rs: clean up downloaded temp files on abort/back, save launcher
  to config only after successful install, auto-start download when a
  preset has an installer_url
- util.rs: add pick_folder() using zenity/kdialog subprocesses (no rfd)
- config.rs: populate installer_url for all 6 built-in presets with
  official download URLs
- Document the Option<Option<Vec<String>>> gamescope pattern at main.rs:307

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

Cargo.toml

@@ -30,6 +30,12 @@ anyhow = "1"
 # XDG config / data paths
 directories = "5"
 
+# Home directory lookup
+dirs = "5"
+
+# Terminal colour output
+owo-colors = "4"
+
 # System tray via D-Bus StatusNotifierItem (KDE, GNOME+AppIndicator, Xfce, etc.)
 ksni = "0.2"