fix: VLM 参数优化 - temperature 0.7, top_p 0.8, 指令放 system+user

- VisionAsync 新增 Temperature=0.7, TopP=0.8
- system prompt 用专业营养识别指令
- userText 用简短"请看图识别食物"配合图片
- 修复重复 prompt 导致 VLM 误读文本的 bug

health_app/lib/pages/diet/diet_capture_page.dart

@@ -2,6 +2,7 @@ import 'dart:io';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:image_picker/image_picker.dart';
+import '../../core/navigation_provider.dart';
 
 final dietProvider = NotifierProvider<DietNotifier, DietState>(DietNotifier.new);
 
@@ -432,7 +433,7 @@ class DietCapturePage extends ConsumerWidget {
             content: Text('饮食记录已保存 ✅'),
             backgroundColor: Color(0xFF635BFF),
           ));
-          Navigator.pop(context);
+          popRoute(ref);
         },
         child: const Text('保存记录'),
         style: ElevatedButton.styleFrom(