fix(mcp): resolve sync-to-other-side functionality failure

This commit implements a three-layer fix to restore the "Sync to other side"
feature when adding or editing MCP servers across Claude and Codex.

Root Cause Analysis:
1. Frontend issue: New MCP entries had their `enabled` field deleted
2. Backend issue: Default value was `false` when `enabled` was missing
3. Core issue: MCP entries were never copied to the other app's servers

Changes:

Frontend (McpFormModal.tsx):
- Set `enabled=true` by default for new MCP entries
- Preserve existing `enabled` state when editing

Backend (services/mcp.rs):
- Change default value from `unwrap_or(false)` to `unwrap_or(true)`
- Implement cross-app MCP replication when `sync_other_side=true`
- Clone MCP entry to other app's servers before syncing to live files

Impact:
- "Sync to Codex" checkbox now correctly adds MCP to both Claude and Codex
- "Sync to Claude" checkbox now correctly adds MCP to both Codex and Claude
- Both config.json and live files (~/.claude.json, ~/.codex/config.toml) are updated
- Fixes regression introduced during project restructure

Tested:
- TypeScript type checking passed
- Rust clippy linting passed
- Manual testing: MCP sync works bidirectionally

src/components/mcp/McpFormModal.tsx

@@ -365,10 +365,13 @@ const McpFormModal: React.FC<McpFormModalProps> = ({
         server: serverSpec,
       };
 
+      // 修复：新增 MCP 时默认启用（enabled=true）
+      // 编辑模式下保留原有的 enabled 状态
       if (initialData?.enabled !== undefined) {
         entry.enabled = initialData.enabled;
-      } else if (!initialData) {
-        delete entry.enabled;
+      } else {
+        // 新增模式：默认启用
+        entry.enabled = true;
       }
 
       const nameTrimmed = (formName || trimmedId).trim();