feat: GUI installers for OnTrack Python and OnTrack-RS

Python installer (ontrack/installer/):
  - ontrack_installer.py: CustomTkinter 4-page wizard
      Welcome → Options → Progress → Done
      Copies app source tree to install dir
      Creates Python venv and pip installs all requirements
      Desktop shortcut: PowerShell .lnk (Windows) / .desktop (Linux/XDG)
      Launch button runs app immediately after install
      --uninstall flag for clean removal
  - installer.spec: PyInstaller spec bundles entire ontrack/ source tree
      into a single self-contained OnTrackInstaller binary
  - build_installer.sh: one-command build (auto-creates temp venv)

Rust installer (ontrack-rs/ontrack-installer/):
  - egui 4-page wizard matching Python installer UX
  - Downloads ontrack + ontrack-gui binaries from GitHub Releases
    OR copies pre-bundled binaries if shipped alongside installer
  - Desktop shortcut: PowerShell .lnk (Windows) / .desktop (Linux/XDG)
  - Async download with live progress bar (Tokio + reqwest)
  - --uninstall flag removes install dir + .desktop entries
  - cargo check: 0 errors

Author: PhaedrusFlow