feat: 启用豆包二遍识别模式以提升实时性和准确率

- 切换到 bigmodel_async endpoint 并启用 enable_nonstream
- 第一遍流式识别提供实时文字预览
- VAD 分句后自动触发第二遍非流式识别提升准确率
- 修改文本处理逻辑从累加改为替换（适配 full 模式）
- 统一配置字段命名：app_key → app_id, access_key → access_token

internal/ws/handler.go

@@ -62,21 +62,21 @@ func (h *Handler) handleConn(c *websocket.Conn) {
   defer close(resultCh)
 
   // Writer goroutine: single writer to avoid concurrent writes
-  // Accumulates all result texts; paste is triggered by stop, not by ASR final.
+  // bigmodel_async with enable_nonstream: server returns full text each time (not incremental)
+  // We replace preview text on each update instead of accumulating.
   var wg sync.WaitGroup
-  var accMu sync.Mutex
-  var accText string
+  var previewMu sync.Mutex
+  var previewText string
   wg.Add(1)
   go func() {
     defer wg.Done()
     for msg := range resultCh {
-      // Accumulate text from both partial and final results
+      // Replace preview text with latest result (full mode)
       if msg.Type == MsgPartial || msg.Type == MsgFinal {
-        accMu.Lock()
-        accText += msg.Text
-        // Send accumulated preview to phone
-        preview := ServerMsg{Type: msg.Type, Text: accText}
-        accMu.Unlock()
+        previewMu.Lock()
+        previewText = msg.Text
+        preview := ServerMsg{Type: msg.Type, Text: previewText}
+        previewMu.Unlock()
         if err := c.WriteMessage(websocket.TextMessage, preview.Bytes()); err != nil {
           log.Warn("ws write error", "err", err)
           return
@@ -128,10 +128,10 @@ func (h *Handler) handleConn(c *websocket.Conn) {
         if active {
           continue
         }
-        // Reset accumulated text for new session
-        accMu.Lock()
-        accText = ""
-        accMu.Unlock()
+        // Reset preview text for new session
+        previewMu.Lock()
+        previewText = ""
+        previewMu.Unlock()
         sa, cl, err := h.asrFactory(resultCh)
         if err != nil {
           log.Error("asr start failed", "err", err)
@@ -154,11 +154,11 @@ func (h *Handler) handleConn(c *websocket.Conn) {
         }
         sendAudio = nil
         active = false
-        // Now paste the accumulated text
-        accMu.Lock()
-        finalText := accText
-        accText = ""
-        accMu.Unlock()
+        // Paste the final preview text
+        previewMu.Lock()
+        finalText := previewText
+        previewText = ""
+        previewMu.Unlock()
         if finalText != "" && h.pasteFunc != nil {
           if err := h.pasteFunc(finalText); err != nil {
             log.Error("auto-paste failed", "err", err)