feat: frontend Actions Tab with structured API, assign/create/remove dialogs (Task 3.3)

- ActionsTab rewritten with master/detail layout (mirrors FiltersTab)
- New AssignActionDialog and CreateActionDialog components
- ActionConfig type extended with active, used_by_jails, source_file, has_local_override
- New API functions: fetchActions, fetchAction, updateAction, createAction, deleteAction, assignActionToJail, removeActionFromJail
- useActionConfig updated to use new structured endpoints
- index.ts barrel exports updated

frontend/src/components/config/__tests__/ActionForm.test.tsx

@@ -24,6 +24,10 @@ const mockConfig: ActionConfig = {
   actionflush: null,
   definition_vars: {},
   init_vars: {},
+  active: false,
+  used_by_jails: [],
+  source_file: "iptables.conf",
+  has_local_override: false,
 };
 
 function renderForm(name: string) {