add(ui/ux): show the running version and build stamp in the header

tests/test_import_flow.py

@@ -1135,3 +1135,10 @@ def test_extra_ebook_extensions_are_configurable(client, monkeypatch):
     finally:
         monkeypatch.delenv("WORDARR_EXTRA_EBOOK_EXTENSIONS")
         importlib.reload(config_module)
+
+
+def test_version_endpoint_reports_a_build_stamp(client):
+    data = client.get("/api/version").json()
+    assert data["version"]
+    # "2026-09-01 15:43" - the newest source file, so a stale deploy stands out
+    assert len(data["built_at"]) == 16 and data["built_at"][4] == "-"

tests/test_ui_dialogs.py

@@ -153,3 +153,9 @@ def test_english_reaches_the_rendered_rows(page):
     page.click("#lang-toggle")
     assert "— überspringen —" in page.locator("#import-table tbody select").first.text_content()
     assert "2,0 KB" in " ".join(page.locator("#import-table .tag.fmt").all_text_contents())
+
+
+def test_the_header_shows_the_running_version(page):
+    stamp = page.locator("#app-version").text_content()
+    assert stamp.startswith("v")
+    assert len(stamp) > 3  # version plus the build stamp behind it