fix(ui): default Sessions page to Active tab, reorder tabs

Active sessions are what engineers care about most. Tab order is now
Active, Prepared, Completed, All.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/SessionHistoryPage.tsx

@@ -21,7 +21,7 @@ export function SessionHistoryPage() {
   const [hasMore, setHasMore] = useState(false)
   const [trees, setTrees] = useState<TreeListItem[]>([])
   const [isLoading, setIsLoading] = useState(true)
-  const [filter, setFilter] = useState<'all' | 'completed' | 'active' | 'prepared'>('all')
+  const [filter, setFilter] = useState<'all' | 'completed' | 'active' | 'prepared'>('active')
 
   // Close session popover state
   const [closingSessionId, setClosingSessionId] = useState<string | null>(null)
@@ -227,7 +227,7 @@ export function SessionHistoryPage() {
 
       {/* Filter Tabs */}
       <div className="mb-6 flex gap-2 border-b border-border">
-        {(['all', 'active', 'completed', 'prepared'] as const).map((tab) => (
+        {(['active', 'prepared', 'completed', 'all'] as const).map((tab) => (
           <button
             key={tab}
             onClick={() => setFilter(tab)}