feat: SAC 0.7.3 custom error pages 401/403/404/429

Centered logo pages with event explanation and allowed actions; router
and apiFetch redirect to them on auth, permission, not-found, and rate-limit errors.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_health.py

@@ -4,6 +4,6 @@ from app.version import APP_NAME, APP_VERSION, APP_VERSION_LABEL
 
 
 def test_version_constants():
-    assert APP_VERSION == "0.7.2"
+    assert APP_VERSION == "0.7.3"
     assert APP_NAME == "Security Alert Center"
-    assert APP_VERSION_LABEL == "Security Alert Center v.0.7.2"
+    assert APP_VERSION_LABEL == "Security Alert Center v.0.7.3"