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Delete the top-level implementation superseded by the gallery/ package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.

tests/test_config.py

@@ -28,7 +28,6 @@ def test_gallery_config_defaults():
     assert cfg.plot_root == GalleryDefaults.plot_root
     assert cfg.cache_enabled == GalleryDefaults.cache_enabled
     assert cfg.inherit_from_parent == GalleryDefaults.inherit_from_parent
-    assert cfg.backup_folder == ""
 
 
 def test_gallery_config_sources_from_dicts():
@@ -47,7 +46,7 @@ def test_gallery_config_sources_invalid_type():
 def test_gallery_config_from_yaml(tmp_path):
     yaml_content = {
         "paths": {"web_folder": "/test/web"},
-        "gallery": {"plot_root": "test_plots", "png_dpi": 200, "backup_folder": "test_backups"},
+        "gallery": {"plot_root": "test_plots", "png_dpi": 200},
         "metadata": {"cache_enabled": False, "inherit_from_parent": False},
         "sources": [
             {"name": "source1", "path": "/path1"},
@@ -64,7 +63,6 @@ def test_gallery_config_from_yaml(tmp_path):
     assert cfg.web_folder == Path("/test/web")
     assert cfg.plot_root == "test_plots"
     assert cfg.png_dpi == 200
-    assert cfg.backup_folder == "test_backups"
     assert cfg.cache_enabled is False
     assert cfg.inherit_from_parent is False
     assert len(cfg.sources) == 2
@@ -106,7 +104,6 @@ def test_gallery_config_to_yaml_round_trip(tmp_path):
         sources=[{"name": "test", "path": "/test"}],
         plot_root="plots",
         png_dpi=300,
-        backup_folder="backups",
     )
 
     yaml_file = tmp_path / "output_config.yaml"
@@ -118,7 +115,6 @@ def test_gallery_config_to_yaml_round_trip(tmp_path):
     assert reloaded.web_folder == cfg.web_folder
     assert reloaded.plot_root == cfg.plot_root
     assert reloaded.png_dpi == cfg.png_dpi
-    assert reloaded.backup_folder == cfg.backup_folder
     assert reloaded.sources[0].name == "test"