refactor: replace skill code with skill ID across the application

- Updated skill handling to use skill IDs instead of skill codes in various components, services, and models.
- Modified tests to reflect changes in skill identification.
- Removed references to skill codes in favor of skill IDs for consistency and clarity.
- Updated localization files to remove skill code references and adjust error messages accordingly.

web/messages/en-US.json

@@ -1612,7 +1612,6 @@
     "refresh": "Refresh",
     "new": "New",
     "filterName": "Filter by name",
-    "filterCode": "Filter by code",
     "searchStatus": "Search statuses",
     "emptyStatus": "No matching statuses",
     "query": "Search",
@@ -1621,8 +1620,6 @@
     "actions": "Actions",
     "loadingRows": "Loading skills...",
     "emptyRows": "No matching skills",
-    "codeRequired": "Enter a skill code.",
-    "codeInvalid": "Skill codes can contain letters, numbers, underscores, and hyphens only.",
     "nameRequired": "Enter a skill name.",
     "instructionRequired": "Enter skill instructions.",
     "editTitle": "Edit",
@@ -1632,8 +1629,6 @@
     "save": "Save",
     "create": "Create",
     "loading": "Loading...",
-    "code": "Code",
-    "codePlaceholder": "Example: refund_skill",
     "name": "Name",
     "namePlaceholder": "Example: Refund Handling",
     "description": "Description",