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/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")