v1.8.3: session pool + sidebar indicators

- SessionPool: LRU cache for SSH/SFTP sessions across server switches
- Sidebar: green dot indicators for servers with active sessions
- Sidebar: active sessions count label
- Terminal: buffer preservation on server switch via get_current_buffer()
- FilesTab/TerminalTab: pool integration for session reuse

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

core/i18n.py

@@ -284,6 +284,7 @@ _EN = {
     "drop_to_download": "Drop to download",
     "recursive_delete_confirm": "Delete folder '{name}' and all contents?",
     "drive": "Drive",
+    "active_sessions": "Active: {count}",
 }
 
 _RU = {
@@ -545,6 +546,7 @@ _RU = {
     "drop_to_download": "Отпустите для скачивания",
     "recursive_delete_confirm": "Удалить папку '{name}' со всем содержимым?",
     "drive": "Диск",
+    "active_sessions": "Активных: {count}",
 }
 
 _ZH = {
@@ -806,6 +808,7 @@ _ZH = {
     "drop_to_download": "释放以下载",
     "recursive_delete_confirm": "删除文件夹 '{name}' 及所有内容?",
     "drive": "驱动器",
+    "active_sessions": "活跃: {count}",
 }
 
 _TRANSLATIONS = {