feat: Configure PWA to launch at songbook with full pre-caching

- Add pwa_start_url to static_html config (launches at songbook.html)
- PWA now caches all 349 songs on install for full offline access
- PWA launches directly at songbook instead of landing page
- Landing page remains available for browser users

This provides a true offline-first experience for campfire singing!

Author: carmiac

book_config.toml

@@ -163,5 +163,6 @@ output_dir = "html"
 stylesheets = ["stylesheets"]
 # song_xslt = "stylesheets/openlyrics.xsl"  # No longer needed with Jinja2 renderer
 image_dir = "img"                        # Directory containing images. This will be copied to the build directory.
+pwa_start_url = "./songbook.html"        # PWA launches at songbook instead of landing page
 
 # Removed - duplicate of bound_pdf