feat: host groups - tree view with folders, drag-free context menu

Data model:
- hosts.json upgraded to {groups: [...], hosts: [...]}, auto-migrates
  old flat-list format on load
- each host gets group field (default '' = ungrouped)

Manager API (core/manager.py):
- add_group / remove_group / rename_group / move_host_to_group
- list_groups / list_hosts_by_group

UI (ui/main_window.py):
- QListWidget replaced with QTreeWidget (folder tree)
- Group nodes: 📁 name (count), expandable/collapsible
- Ungrouped always shown as 📭 未分组
- Right-click context menu:
  - Empty area / group: New group, Rename, Delete
  - Host: Connect, Edit, Delete, Move to group (submenu)
- Double-click host = connect, double-click group = expand/collapse
- All 6 test suites pass

core/manager.py

@@ -17,12 +17,13 @@ AI_CONFIG_FILE = CONFIG_DIR / "ai.json"
 
 
 class ConnectionManager:
-    """多主机连接管理 + 配置持久化"""
+    """多主机连接管理 + 配置持久化 + 分组"""
 
     def __init__(self):
         CONFIG_DIR.mkdir(parents=True, exist_ok=True)
         self._lock = threading.RLock()
         self.hosts: List[dict] = []  # 主机配置
+        self.groups: List[str] = []  # 分组名称列表（保持顺序）
         self.connections: Dict[str, SSHConnection] = {}  # host_id -> SSHConnection
         self._load_hosts()
 
@@ -30,23 +31,91 @@ class ConnectionManager:
         if CONFIG_FILE.exists():
             try:
                 with open(CONFIG_FILE, "r", encoding="utf-8") as f:
-                    self.hosts = json.load(f)
+                    data = json.load(f)
+                if isinstance(data, list):
+                    # 旧格式：flat list，自动迁移
+                    self.hosts = data
+                    self.groups = []
+                elif isinstance(data, dict):
+                    self.hosts = data.get("hosts", [])
+                    self.groups = data.get("groups", [])
             except Exception:
                 self.hosts = []
+                self.groups = []
         if not self.hosts:
-            # 给一个示例条目，让 UI 不为空
             self.hosts = [{
                 "id": "demo", "name": "示例主机", "host": "127.0.0.1",
                 "port": 22, "username": "root", "password": "", "key_path": "",
+                "group": "",
             }]
+        # 给所有旧主机补 group 字段
+        for h in self.hosts:
+            if "group" not in h:
+                h["group"] = ""
 
     def save_hosts(self):
+        data = {"groups": self.groups, "hosts": self.hosts}
         with open(CONFIG_FILE, "w", encoding="utf-8") as f:
-            json.dump(self.hosts, f, ensure_ascii=False, indent=2)
+            json.dump(data, f, ensure_ascii=False, indent=2)
 
     def list_hosts(self) -> List[dict]:
         return list(self.hosts)
 
+    def list_groups(self) -> List[str]:
+        """返回分组名列表（不含「未分组」）"""
+        return list(self.groups)
+
+    def add_group(self, name: str) -> bool:
+        """新建分组；返回是否成功"""
+        name = name.strip()
+        if not name or name in self.groups:
+            return False
+        self.groups.append(name)
+        self.save_hosts()
+        return True
+
+    def remove_group(self, name: str):
+        """删除分组；组内主机移到未分组"""
+        if name in self.groups:
+            self.groups.remove(name)
+        for h in self.hosts:
+            if h.get("group") == name:
+                h["group"] = ""
+        self.save_hosts()
+
+    def rename_group(self, old_name: str, new_name: str) -> bool:
+        if old_name not in self.groups:
+            return False
+        new_name = new_name.strip()
+        if not new_name or new_name in self.groups:
+            return False
+        idx = self.groups.index(old_name)
+        self.groups[idx] = new_name
+        for h in self.hosts:
+            if h.get("group") == old_name:
+                h["group"] = new_name
+        self.save_hosts()
+        return True
+
+    def move_host_to_group(self, host_id: str, group_name: str):
+        """移动主机到指定分组"""
+        for h in self.hosts:
+            if h.get("id") == host_id:
+                h["group"] = group_name
+                self.save_hosts()
+                return
+
+    def list_hosts_by_group(self) -> dict:
+        """返回 {group_name: [hosts]}，"" 为未分组"""
+        result = {g: [] for g in self.groups}
+        result[""] = []  # 未分组
+        for h in self.hosts:
+            g = h.get("group", "")
+            if g not in result:
+                result[g] = []
+            result[g].append(dict(h))
+        return result
+
     def get_host(self, host_id: str) -> Optional[dict]:
         for h in self.hosts:
             if h.get("id") == host_id: