Fix briefing settings not loading when tab is active on mount

The watch(activeTab) only fires on changes, not the initial value.
When localStorage had settings_tab="briefing", onMounted skipped
loadBriefingTab() so the form always showed default empty values.
Follows the same pattern already used for the groups panel (line 338).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/SettingsView.vue

@@ -336,6 +336,7 @@ onMounted(async () => {
       // base URL not configured yet
     }
     if (activeTab.value === "groups") loadGroupsPanel();
+    if (activeTab.value === "briefing") loadBriefingTab();
   }
 });