v1.0.0 — BookVault 1.0: now a native desktop app, too

BookVault reaches 1.0 with a third way to run it: a native desktop app for
macOS, Windows, and Linux, alongside the local web app and the MCP server. All
three are front-ends over the same backend -- the desktop app embeds the exact
bookvault-web server and shows it in a native window, with no browser tab or
terminal needed.

Added

• bookvault-desktop -- a native desktop app. A
  pywebview shell that starts the FastAPI web
  app on a private 127.0.0.1 port in a background thread and shows it in a
  native OS window (WKWebView on macOS, WebView2 on Windows, WebKitGTK on
  Linux). It reuses bookvault-web verbatim -- the backend is imported, not
  duplicated -- so the web app, the MCP server, and their Docker images are
  unchanged. A splash is shown while a saved session restores (which drives a
  real headless Chromium), then the live app loads; closing the window cleanly
  stops the backend and the Playwright/Chromium session (bounded graceful
  shutdown, so the browser is never orphaned).

  .venv/bin/pip install -e ./core -e ./web -e ./desktop
  .venv/bin/bookvault-desktop

Notes

• The desktop app currently runs from a source checkout; packaged installers
  (.dmg / .msi / .AppImage) and package-manager channels (Homebrew /
  winget / AUR) are the next step.
• tests/test_desktop.py skips itself unless the desktop package is
  installed, so the released web/MCP CI is unaffected.