feat: multi-connection parallel support

- connection store: connectedIds array, per-connection health/serverInfo maps
- connect() no longer disconnects existing connections
- New switchTo(id), disconnectById(id), isConnectedById(id) methods
- Per-connection health check timers (Map-based)
- ConnectionCard: switch to connected, connect if not, disconnect if active
- MainArea: per-connection tab state (save/restore on switch)
- Connection switcher dropdown in tab bar showing all open connections
- Clean up tab state when connection disconnects

ROADMAP.md

@@ -17,7 +17,7 @@
 - [x] **命令日志** — 记录每次命令的耗时/连接/时间，最多 5000 条，只写模式过滤
 - [x] **自动更新** — electron-updater 集成，启动检查，下载进度，发行说明
 - [x] **多标签页** — 单连接支持同时打开 Status/Keys/CLI/SlowLog/内存分析/批量删除 多个标签
-- [ ] **多连接并行** — 同时打开多个连接，各自独立标签页
+- [x] **多连接并行** — 同时打开多个连接，各自独立标签页
 - [ ] **Monaco 编辑器** — 替换 StringEditor 的 textarea，支持 JSON 折叠/展开/语法高亮
 - [ ] **Redis 命令自动补全** — CLI 输入时智能提示（基于 commands.js 命令数据库）
 - [ ] **CLI SUBSCRIBE/MONITOR** — 实时消息流 + MONITOR 命令流 + 停止按钮