feat(web): customizable placeholder, footer text, and color theme via BOR_* env vars

BOR_INPUT_PLACEHOLDER / BOR_FOOTER_TEXT / BOR_THEME (+ the indigo.css example theme); authoring guide: frontend/assets/themes/README.md, docs: README 'Customizing the look'.

tests/unit/test_save_as_doc_button.py

@@ -51,7 +51,12 @@ def test_app_config_dict_carries_the_docs_flag() -> None:
 
     s = _settings()
     body = app_config(s)
-    assert set(body) == {"app_name", "version", "docs_repo_configured"}
+    # Phase 62 (task 01): the response grew to the six-key set — the
+    # phase-62 UI customization keys ride the SAME endpoint.
+    assert set(body) == {
+        "app_name", "version", "docs_repo_configured",
+        "input_placeholder", "footer_text", "theme",
+    }
     assert body["docs_repo_configured"] is s.docs_configured
     assert body["docs_repo_configured"] is False