feat(mcp): implement unified MCP management for v3.7.0

BREAKING CHANGE: Migrate from app-specific MCP storage to unified structure

## Phase 1: Data Structure Migration
- Add McpApps struct with claude/codex/gemini boolean fields
- Add McpServer struct for unified server definition
- Add migration logic in MultiAppConfig::migrate_mcp_to_unified()
- Integrate automatic migration into MultiAppConfig::load()
- Support backward compatibility through Optional fields

## Phase 2: Backend Services Refactor
- Completely rewrite services/mcp.rs for unified management:
  * get_all_servers() - retrieve all MCP servers
  * upsert_server() - add/update unified server
  * delete_server() - remove server
  * toggle_app() - enable/disable server for specific app
  * sync_all_enabled() - sync to all live configs
- Add single-server sync functions to mcp.rs:
  * sync_single_server_to_claude/codex/gemini()
  * remove_server_from_claude/codex/gemini()
- Add read_mcp_servers_map() to claude_mcp.rs and gemini_mcp.rs
- Add new Tauri commands to commands/mcp.rs:
  * get_mcp_servers, upsert_mcp_server, delete_mcp_server
  * toggle_mcp_app, sync_all_mcp_servers
- Register new commands in lib.rs

## Migration Strategy
- Detects old structure (mcp.claude/codex/gemini.servers)
- Merges into unified mcp.servers with apps markers
- Handles conflicts by merging enabled apps
- Clears old structures after migration
- Saves migrated config automatically

## Known Issues
- Old commands still need compatibility layer (WIP)
- toml_edit type conversion needs fixing (WIP)
- Frontend not yet implemented (Phase 3 pending)

Related: v3.6.2 -> v3.7.0

src-tauri/src/claude_mcp.rs

@@ -231,6 +231,27 @@ pub fn validate_command_in_path(cmd: &str) -> Result<bool, AppError> {
     Ok(false)
 }
 
+/// 读取 ~/.claude.json 中的 mcpServers 映射
+pub fn read_mcp_servers_map() -> Result<std::collections::HashMap<String, Value>, AppError> {
+    let path = user_config_path();
+    if !path.exists() {
+        return Ok(std::collections::HashMap::new());
+    }
+
+    let root = read_json_value(&path)?;
+    let servers = root
+        .get("mcpServers")
+        .and_then(|v| v.as_object())
+        .map(|obj| {
+            obj.iter()
+                .map(|(k, v)| (k.clone(), v.clone()))
+                .collect()
+        })
+        .unwrap_or_default();
+
+    Ok(servers)
+}
+
 /// 将给定的启用 MCP 服务器映射写入到用户级 ~/.claude.json 的 mcpServers 字段
 /// 仅覆盖 mcpServers，其他字段保持不变
 pub fn set_mcp_servers_map(