feat(scenarios): overhaul export/import with multi-entity YAML format

- export now accepts includeEnvironment and credentialIds query params
- output is a YAML stream with comment-delimited environment, credential, and scenario sections
- scenario entity includes credentials array with alias mappings
- import accepts both old single-object and new array format, upserts envs/creds before linking
- new ExportScenarioModal with env and per-credential checkboxes replaces direct download

client/src/i18n/locales/en.json

@@ -181,7 +181,12 @@
     "run_modal_title": "Select environment",
     "run_modal_env_label": "Environment",
     "run_modal_no_env": "No environments found. Create an environment before running a scenario.",
-    "run_modal_env_required": "Please select an environment"
+    "run_modal_env_required": "Please select an environment",
+    "export_modal_title": "Export Scenario",
+    "export_include_environment": "Include Environment",
+    "export_include_credentials": "Include Credentials",
+    "field_name": "Name",
+    "field_id": "ID"
   },
   "theme": {
     "switch_to_light": "Switch to light theme",