Fix stale identifier flags: refresh composable on Settings toggle

The identifier-flags composable fetched settings once and cached them in a
module singleton, so toggling an identifier in Settings (gauge/maintenance/FQDN
per asset type) did not take effect on already-open asset views until a full
page reload. Disabled identifiers kept showing.

- identifierSettings.js: extract applySetting/fetchFlags; export
  reloadIdentifierFlags() and setIdentifierFlag(name, assettype, enabled) to
  mutate the shared reactive state.
- SystemSettings.vue: push each successful matrix toggle into the shared state
  via setIdentifierFlag so dependent views react immediately.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/views/settings/SystemSettings.vue

@@ -423,6 +423,7 @@
 <script setup>
 import { ref, reactive, onMounted, computed } from 'vue'
 import { settingsApi } from '../../api'
+import { setIdentifierFlag } from '../../composables/identifierSettings'
 
 const settings = reactive({
   // Zabbix
@@ -577,6 +578,9 @@ async function toggleIdentifier(name, assettype) {
       }
     }
     identifierMatrix[key] = newValue
+    // Push into the shared composable so open asset views react without a
+    // full page reload (the composable otherwise fetches only once).
+    setIdentifierFlag(name, assettype, newValue)
     success.value = 'Setting saved'
     setTimeout(() => { success.value = '' }, 2000)
   } catch (e) {