Remove pre-package-restructure cruft and dead features

Delete the top-level implementation superseded by the gallery/ package
conversion (generate_gallery.py, orchestration/, root templates/ and
assets/, python/ scripts), stray scratch files, and docs describing a
container/GitLab-CI coverage workflow that no longer exists. Also drop
two half-wired, never-invoked features: the backup_folder config/TUI
option (create_backup() was never called from the pipeline) and the
unfinished export-to-LaTeX JS/CSS. Update README's install instructions
to the current Gitea remote.

gallery/assets/css/main.css

@@ -18,7 +18,6 @@
 @import url('./metadata.css');
 @import url('./metadata-section.css');
 @import url('./folder-metadata.css');
-@import url('./export.css');
 
 /* View controls - must come after grid.css to override */
 @import url('./view-controls.css');