fix(mcp): initialize McpRoot with v3.7.0 unified structure by default

Problem:
- On first launch, McpRoot::default() created `servers: None`
- McpService::get_all_servers() would incorrectly return error "old structure detected"
- This confused new users who had no legacy MCP data

Root Cause:
- Derived Default trait sets Option<T> fields to None
- New installations should start with v3.7.0 structure immediately

Solution:
- Explicitly implement Default for McpRoot
- Initialize `servers: Some(HashMap::new())` for v3.7.0+ structure
- Legacy fields (claude/codex/gemini) remain empty, only used for deserializing old configs

Impact:
- First-time users get correct v3.7.0 structure immediately
- migrate_mcp_to_unified() correctly detects already-migrated state
- No false "old structure" errors on fresh installs

src-tauri/src/app_config.rs

@@ -86,7 +86,7 @@ impl McpConfig {
 }
 
 /// MCP 根配置（v3.7.0 新旧结构并存）
-#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct McpRoot {
     /// 统一的 MCP 服务器存储（v3.7.0+）
     #[serde(skip_serializing_if = "Option::is_none")]
@@ -101,6 +101,19 @@ pub struct McpRoot {
     pub gemini: McpConfig,
 }
 
+impl Default for McpRoot {
+    fn default() -> Self {
+        Self {
+            // v3.7.0+ 默认使用新的统一结构（空 HashMap）
+            servers: Some(HashMap::new()),
+            // 旧结构保持空，仅用于反序列化旧配置时的迁移
+            claude: McpConfig::default(),
+            codex: McpConfig::default(),
+            gemini: McpConfig::default(),
+        }
+    }
+}
+
 /// Prompt 配置：单客户端维度
 #[derive(Debug, Clone, Serialize, Deserialize, Default)]
 pub struct PromptConfig {