feat(desktop): add cron tool to Tools page UI

- Add 'group:cron' to TOOL_GROUPS in IPC handler and agent tools
- Add cron tool description and group name to use-tools hook
- Add Time04Icon for cron group in tool-list component
- Add subagent group icon (UserMultipleIcon) for completeness

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/tools/groups.ts

@@ -36,6 +36,9 @@ export const TOOL_GROUPS: Record<string, string[]> = {
   // Subagent tools
   "group:subagent": ["sessions_spawn"],
 
+  // Cron/scheduling tools
+  "group:cron": ["cron"],
+
   // All core tools
   "group:core": [
     "read",