feat(web): MD3 矢量图标组件 + 全量 emoji 替换为新图标

新增 src/components/Icon.tsx（Material Symbols Rounded 风格，24 viewBox / currentColor /
零运行时依赖 / 可 tree-shake / 离线优先），导出 <Icon name size /> 与 IconName 类型，
覆盖全部现有界面图标的语义映射（chart/dna/chat/memory/docs/plugin/channel/device/robot/
clock/agent/night/trend/users/settings/light/ac/mic/micOff/sun/moon/lock/block/check/close/
gear/file/package/folder/shield/storage/refresh/vision/home/arrow/terminal）。

将用户端（Nav/TopBar/ChatInput/chat + 各 modes）与管理面板（sidebar 15 模块/Dashboard
卡片/System SectionTitle/Observability StatCard/Agents/Backthink/Automations/AdminTable
empty）中的 emoji 图标全部替换为 <Icon name=... />，icon 字段由 string 改为 IconName /
ReactNode。保留正文说明性箭头（如“设置→连接”）与流程方向箭头。

npm run build（tsc --noEmit + vite build）通过：79 模块，JS 300KB/gzip 90KB。

frontend/web/src/admin/sections.tsx

@@ -1,10 +1,11 @@
 import type { AdminSectionId } from "./router";
+import type { IconName } from "../components/Icon";
 
 /** Sidebar grouping for the 15 admin modules. Groups are visual only. */
 export interface AdminSectionMeta {
   id: AdminSectionId;
   label: string;
-  icon: string;
+  icon: IconName;
   group: string;
 }
 
@@ -13,21 +14,21 @@ export interface AdminSectionMeta {
  * the section headers. All are clickable — several are shells until M10+ backend.
  */
 export const ADMIN_SECTIONS: ReadonlyArray<AdminSectionMeta> = [
-  { id: "dashboard", label: "仪表盘", icon: "📊", group: "总览" },
-  { id: "personas", label: "人格管理", icon: "🧬", group: "内容" },
-  { id: "sessions", label: "会话管理", icon: "💬", group: "内容" },
-  { id: "memories", label: "记忆管理", icon: "🧠", group: "内容" },
-  { id: "rag", label: "知识库 RAG", icon: "📚", group: "内容" },
-  { id: "plugins", label: "插件与能力", icon: "🔌", group: "能力与连接" },
-  { id: "channels", label: "渠道管理", icon: "📡", group: "能力与连接" },
-  { id: "iot", label: "IoT 设备", icon: "📲", group: "能力与连接" },
-  { id: "llm", label: "LLM 模型", icon: "🤖", group: "能力与连接" },
-  { id: "automations", label: "自动化与提醒", icon: "⏰", group: "能力与连接" },
-  { id: "agents", label: "Agent 监控", icon: "🧩", group: "运行时" },
-  { id: "backthink", label: "后台思考", icon: "🌙", group: "运行时" },
-  { id: "observability", label: "可观测性", icon: "📈", group: "运行时" },
-  { id: "users", label: "用户与权限", icon: "👥", group: "治理" },
-  { id: "system", label: "系统设置", icon: "⚙", group: "治理" },
+  { id: "dashboard", label: "仪表盘", icon: "chart", group: "总览" },
+  { id: "personas", label: "人格管理", icon: "dna", group: "内容" },
+  { id: "sessions", label: "会话管理", icon: "chat", group: "内容" },
+  { id: "memories", label: "记忆管理", icon: "memory", group: "内容" },
+  { id: "rag", label: "知识库 RAG", icon: "docs", group: "内容" },
+  { id: "plugins", label: "插件与能力", icon: "plugin", group: "能力与连接" },
+  { id: "channels", label: "渠道管理", icon: "channel", group: "能力与连接" },
+  { id: "iot", label: "IoT 设备", icon: "device", group: "能力与连接" },
+  { id: "llm", label: "LLM 模型", icon: "robot", group: "能力与连接" },
+  { id: "automations", label: "自动化与提醒", icon: "clock", group: "能力与连接" },
+  { id: "agents", label: "Agent 监控", icon: "agent", group: "运行时" },
+  { id: "backthink", label: "后台思考", icon: "night", group: "运行时" },
+  { id: "observability", label: "可观测性", icon: "trend", group: "运行时" },
+  { id: "users", label: "用户与权限", icon: "users", group: "治理" },
+  { id: "system", label: "系统设置", icon: "settings", group: "治理" },
 ];
 
 /** Group headers in sidebar order (derived from ADMIN_SECTIONS). */