Test del helper di salvataggio immagine e fix pulizia backslash nel nome

Refs #119

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/frontend/flask_app/tests/test_save_image_js.py

@@ -0,0 +1,35 @@
+"""Runs the plain-node save-image helper test (tests/js/test_save_image.js).
+
+Same thin pytest wrapper pattern as test_vision_overlay_js.py: shells out to
+``node`` and asserts the script's own exit code. Skips (does not fail) when
+node is not installed.
+
+Covers gitea:Adriano/TieMeasureFlow#119.
+"""
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+FLASK_APP_DIR = Path(__file__).resolve().parent.parent
+JS_TEST = Path(__file__).resolve().parent / "js" / "test_save_image.js"
+
+
+def test_save_image_js_behaviour():
+    node = shutil.which("node")
+    if not node:
+        pytest.skip("node non installato: test JS del salvataggio immagine saltato")
+
+    result = subprocess.run(
+        [node, str(JS_TEST)],
+        cwd=str(FLASK_APP_DIR),
+        capture_output=True,
+        text=True,
+        timeout=30,
+    )
+
+    assert result.returncode == 0, (
+        f"tests/js/test_save_image.js fallito (exit {result.returncode})\n"
+        f"stdout:\n{result.stdout}\nstderr:\n{result.stderr}"
+    )