fix(aria): collapse filter sidebar by default on all pages

PrecursorAlerts and ThreatEvents were still starting expanded.

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

frontend/src/components/PrecursorAlerts.svelte

@@ -14,7 +14,7 @@
   let resolving: number | null = null;
 
   // Filter sidebar
-  let filterSidebarCollapsed = false;
+  let filterSidebarCollapsed = true;
   let filterStatus: 'active' | 'resolved' | 'all' = 'active';
   let filterType: SequenceType | '' = '';
   let filterDevice = '';

frontend/src/components/ThreatEvents.svelte

@@ -16,7 +16,7 @@
   let severityFilter: ThreatSeverity | '' = initialSeverity;
   let deviceSearch = '';
   let deviceSearchInput = '';
-  let filterSidebarCollapsed = false;
+  let filterSidebarCollapsed = true;
   let searchTimer: ReturnType<typeof setTimeout>;
 
   $: hasActiveFilters = !!severityFilter || !!deviceSearch;