feat: 智能体精选排序与新对话智能体选择器

- 后端新增 featured_order 字段及精选排序 API（/agents/selector、/agents/{id}/featured、/agents/featured/order），移除 user_agent_pin 表
- 前端新对话页 (ChatPage) 与输入框 (InputBox) 接入 agent-selector，选中后跳转 AgentChatPage 并通过 sessionStorage 透传首条消息
- 新增管理员智能体管理弹窗 (agent-manage-dialog) 支持精选添加/移除/上下移动

frontend-web/src/core/agents/api.ts

@@ -1,7 +1,12 @@
 import { getBackendBaseURL } from "@/core/config";
 import { apiFetch } from "@/core/api";
 
-import type { Agent, CreateAgentRequest, UpdateAgentRequest } from "./types";
+import type {
+  Agent,
+  AgentSelectorResponse,
+  CreateAgentRequest,
+  UpdateAgentRequest,
+} from "./types";
 
 const BACKEND_UNAVAILABLE_STATUSES = new Set([502, 503, 504]);
 
@@ -75,6 +80,50 @@ export async function deleteAgent(id: string): Promise<void> {
   if (!res.ok) throw new Error(`Failed to delete agent: ${res.statusText}`);
 }
 
+export async function getAgentSelector(): Promise<AgentSelectorResponse> {
+  const res = await apiFetch(`${getBackendBaseURL()}/api/agents/selector`);
+  if (!res.ok) throw new Error(`Failed to load agent selector: ${res.statusText}`);
+  return res.json() as Promise<AgentSelectorResponse>;
+}
+
+export async function reorderFeaturedAgents(agentIds: string[]): Promise<void> {
+  const res = await apiFetch(
+    `${getBackendBaseURL()}/api/agents/featured/order`,
+    {
+      method: "PUT",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ agent_ids: agentIds }),
+    },
+  );
+  if (!res.ok) {
+    const err = (await res.json().catch(() => ({}))) as { detail?: string };
+    throw new Error(
+      err.detail ?? `Failed to reorder featured agents: ${res.statusText}`,
+    );
+  }
+}
+
+export async function setAgentFeatured(
+  id: string,
+  order: number | null,
+): Promise<Agent> {
+  const res = await apiFetch(
+    `${getBackendBaseURL()}/api/agents/${id}/featured`,
+    {
+      method: "PUT",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ order }),
+    },
+  );
+  if (!res.ok) {
+    const err = (await res.json().catch(() => ({}))) as { detail?: string };
+    throw new Error(
+      err.detail ?? `Failed to set featured: ${res.statusText}`,
+    );
+  }
+  return res.json() as Promise<Agent>;
+}
+
 export async function checkAgentName(
   name: string,
 ): Promise<{ available: boolean; name: string }> {