feat: 实现关键帧检测与成本控制

- EdgeProcessor：sampleFrame 降采样 160x120 + compareFrames 像素差异对比
- 重复画面跳过：similarity > 0.9 时不发送 query
- 混合采样策略：静默 5s / 用户说话 1s（docs/08-成本控制.md）
- 请求统计：queryCount + totalTokens 实时显示
- App 状态栏显示成本统计

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

frontend/src/hooks/useVisionSession.ts

@@ -14,18 +14,27 @@ import { showToast } from "../lib/toast";
 import { loadConfig, saveConfig } from "../lib/storage";
 import { useCamera } from "../components/CameraManager";
 import { useMicrophone } from "../components/MicManager";
-import { useVAD } from "../components/EdgeProcessor";
+import { useVAD, sampleFrame, compareFrames } from "../components/EdgeProcessor";
 import { useWebSocketManager } from "../components/WebSocketManager";
 import type { ChatMessage, SessionConfig, ServerMessage, LLMDoneMessage } from "../types";
 
 const MAX_HISTORY_ROUNDS = 10;
 
+export interface SessionStats {
+  queryCount: number;
+  totalTokens: number;
+}
+
 export function useVisionSession() {
   const [messages, setMessages] = useState<ChatMessage[]>([]);
   const [currentReply, setCurrentReply] = useState<string>("");
   const [isProcessing, setIsProcessing] = useState(false);
   const [isAudioPlaying, setIsAudioPlaying] = useState(false);
   const [config, setConfig] = useState<SessionConfig>(loadConfig);
+  const [stats, setStats] = useState<SessionStats>({ queryCount: 0, totalTokens: 0 });
+
+  // 上一帧采样数据（用于关键帧检测）
+  const prevFrameRef = useRef<Uint8ClampedArray | null>(null);
 
   // 对话历史（role + content），用于多轮上下文
   const historyRef = useRef<Array<{ role: string; content: string }>>([]);
@@ -107,6 +116,23 @@ export function useVisionSession() {
           return;
         }
 
+        // 关键帧检测：与上一帧对比，相似度过高则跳过
+        const video = videoRef.current;
+        if (video) {
+          const currentSample = sampleFrame(video);
+          if (currentSample && prevFrameRef.current) {
+            const { similarity } = compareFrames(prevFrameRef.current, currentSample);
+            if (similarity > 0.9) {
+              console.log(`[Session] 画面无变化 (similarity=${similarity.toFixed(2)})，跳过`);
+              prevFrameRef.current = currentSample;
+              return;
+            }
+          }
+          if (currentSample) {
+            prevFrameRef.current = currentSample;
+          }
+        }
+
         const requestId = uuidv4();
         send({
           type: "query",
@@ -115,6 +141,9 @@ export function useVisionSession() {
           audio: encodeAudioToBase64(audio),
         });
 
+        // 更新请求统计
+        setStats((prev) => ({ ...prev, queryCount: prev.queryCount + 1 }));
+
         // 添加用户消息（STT 流式结果会逐步更新文本）
         setMessages((prev) => [
           ...prev,
@@ -122,7 +151,7 @@ export function useVisionSession() {
         ]);
         setIsProcessing(true);
       },
-      [captureFrame, send]
+      [captureFrame, send, videoRef]
     ),
   });
 
@@ -159,6 +188,14 @@ export function useVisionSession() {
             historyRef.current = historyRef.current.slice(-MAX_HISTORY_ROUNDS * 2);
           }
 
+          // 累计 token 统计
+          if (done.tokens_used?.total) {
+            setStats((prev) => ({
+              ...prev,
+              totalTokens: prev.totalTokens + done.tokens_used.total,
+            }));
+          }
+
           setMessages((prev) => [
             ...prev,
             {
@@ -222,7 +259,9 @@ export function useVisionSession() {
     setMessages([]);
     setCurrentReply("");
     setIsProcessing(false);
+    setStats({ queryCount: 0, totalTokens: 0 });
     historyRef.current = [];
+    prevFrameRef.current = null;
   }, [stopVAD, stopMic, stopCamera, disconnect]);
 
   /** 打断当前回复 */
@@ -257,6 +296,7 @@ export function useVisionSession() {
     stream,
     config,
     updateConfig,
+    stats,
     startSession,
     stopSession,
     interrupt,