v1.8.56: Database Tree Explorer + thread-safe SQL operations

- Add HeidiSQL-style database tree panel (Databases → Tables → Columns)
- Lazy loading with ttk.Treeview, context menus, double-click SELECT TOP 1000
- Fix pymysql thread safety: serialize all DB ops with threading.Lock
- Use lock.acquire(timeout=10) to prevent deadlocks between tree and query threads
- Always reset _executing flag in finally block to prevent stuck queries
- Add _ensure_connected() auto-reconnect on broken connections
- Add sql_client check_connection() null safety
- Add 12 tree-related i18n keys (EN/RU/ZH)
- Clean up old releases

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/i18n.py

@@ -327,6 +327,19 @@ _EN = {
     "query_connected": "Connected to {alias} ({db})",
     "query_connecting": "Connecting...",
     "query_disconnected": "Not connected",
+
+    # Database tree
+    "tree_databases": "Databases",
+    "tree_refresh": "Refresh",
+    "tree_use_db": "Use database",
+    "tree_select_top": "SELECT TOP 1000",
+    "tree_describe": "Describe table",
+    "tree_copy_name": "Copy name",
+    "tree_select_column": "Insert column",
+    "tree_loading": "Loading...",
+    "tree_no_tables": "(no tables)",
+    "tree_no_columns": "(no columns)",
+    "tree_connected": "Connected",
     "query_exported": "Exported to {path}",
 
     # Redis tab
@@ -741,6 +754,19 @@ _RU = {
     "query_disconnected": "Не подключено",
     "query_exported": "Экспортировано в {path}",
 
+    # Database tree
+    "tree_databases": "Базы данных",
+    "tree_refresh": "Обновить",
+    "tree_use_db": "Выбрать базу",
+    "tree_select_top": "SELECT TOP 1000",
+    "tree_describe": "Описание таблицы",
+    "tree_copy_name": "Копировать имя",
+    "tree_select_column": "Вставить колонку",
+    "tree_loading": "Загрузка...",
+    "tree_no_tables": "(нет таблиц)",
+    "tree_no_columns": "(нет колонок)",
+    "tree_connected": "Подключено",
+
     # Redis tab
     "redis_clear": "Очистить",
     "redis_execute": "Выполнить",
@@ -1153,6 +1179,19 @@ _ZH = {
     "query_disconnected": "未连接",
     "query_exported": "已导出到 {path}",
 
+    # Database tree
+    "tree_databases": "数据库",
+    "tree_refresh": "刷新",
+    "tree_use_db": "选择数据库",
+    "tree_select_top": "SELECT TOP 1000",
+    "tree_describe": "表描述",
+    "tree_copy_name": "复制名称",
+    "tree_select_column": "插入列",
+    "tree_loading": "加载中...",
+    "tree_no_tables": "(无表)",
+    "tree_no_columns": "(无列)",
+    "tree_connected": "已连接",
+
     # Redis tab
     "redis_clear": "清除",
     "redis_execute": "执行",

core/sql_client.py

@@ -70,6 +70,8 @@ class SQLClient:
             log.info("sql_client: disconnected")
 
     def check_connection(self) -> bool:
+        if self._conn is None:
+            return False
         try:
             cur = self._conn.cursor()
             cur.execute("SELECT 1")