v1.2.0 — Linux installer (AppImage)

Adds a Linux AppImage, completing the desktop trio (macOS / Windows / Linux).
Built with PyInstaller + appimagetool on an ubuntu-latest runner, with a
headless smoke test (xvfb) that boots the frozen app to prove the GTK/WebKit
binding works, then attaches the .AppImage to each release.

Added

• Linux .AppImage. chmod +x and run it. It bundles the app + the gi
  bindings/typelib; WebKitGTK 4.1 is a host runtime dependency (WebKit's
  multiprocess helpers use compile-time absolute paths that can't live inside an
  AppImage) — one apt line covers it on Ubuntu 24.04+
  (libwebkit2gtk-4.1-0 gir1.2-webkit2-4.1 gir1.2-gtk-3.0). Chromium is fetched
  on first run; app data lives in ~/.local/share/BookVault/.
• CI (.github/workflows/desktop-linux.yml) builds + smoke-tests the AppImage on
  ubuntu-latest on every tag and attaches it to the GitHub Release.