feat: agent avatars, task tags, search, edit/delete agents, cron readability, timezone fix

- Agent avatars: first-letter + deterministic color avatar in all views
- Agent management: edit (name/description) and delete with confirmation
- Execution API: validates agent existence, returns 410 if deleted
- Task tags: JSON array field, editable with tag pills UI
- Task search: by name (?q=) and by tags (?tags=) and by status
- Task edit: simplified to name/tags/description only (not agent params)
- Cron display: human-readable Chinese (e.g. '每5分钟', '每天 09:00')
- Timezone fix: UTC→local via dayjs.utc().local() across all pages
- Content area: full width (removed max-width:1200px constraints)
- Login page: standalone layout without sidebar

backend/app/services/agent.py

@@ -52,3 +52,11 @@ class AgentService:
         stmt = select(func.count(ScheduledTask.id)).where(ScheduledTask.agent_id == agent_id)
         result = await self.db.execute(stmt)
         return result.scalar() or 0
+
+    async def delete(self, agent_id: int) -> bool:
+        agent = await self.get(agent_id)
+        if agent is None:
+            return False
+        await self.db.delete(agent)
+        await self.db.flush()
+        return True