perf: optimize /api/sessions endpoint response payload

Slim down SessionSummary responses to reduce payload size:
- Replace full todos array with computed todoProgress summary
- Replace metadata object with only necessary fields (name, path, summary.text)
- Remove unused fields (thinking, createdAt, permissionMode, modelMode)
- Refactor sorting to work on full Session objects before mapping

This significantly reduces /api/sessions response size, especially for
sessions with many todos or tools.

web/src/components/SessionList.tsx

@@ -58,11 +58,9 @@ function getSessionTitle(session: SessionSummary): string {
 }
 
 function getTodoProgress(session: SessionSummary): { completed: number; total: number } | null {
-    if (!session.todos || session.todos.length === 0) return null
-    const total = session.todos.length
-    const completed = session.todos.filter(t => t.status === 'completed').length
-    if (completed === total) return null
-    return { completed, total }
+    if (!session.todoProgress) return null
+    if (session.todoProgress.completed === session.todoProgress.total) return null
+    return session.todoProgress
 }
 
 export function SessionList(props: {