fix: SSH keepalive + auto-reconnect for SFTP and terminal

- Add transport.set_keepalive(30) to all SSH connections
- Add SFTPSession.reconnect() method
- Auto-reconnect in _refresh_remote with retry on failure
- Auto-reconnect before upload/download operations
- Guard all remote navigation methods with connection checks
- ShellSession.connected wrapped in try/except
- Terminal: reset reconnect counter on send failure, increase max to 5
- Terminal: trigger reconnect on send to dead session

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

gui/tabs/terminal_tab.py

@@ -17,7 +17,7 @@ class TerminalTab(ctk.CTkFrame):
         self._current_alias: str | None = None
         self._session: ShellSession | None = None
         self._reconnect_count = 0
-        self._max_reconnect = 3
+        self._max_reconnect = 5
         self._intentional_disconnect = False
 
         # Import here to avoid circular issues
@@ -142,6 +142,10 @@ class TerminalTab(ctk.CTkFrame):
         session = self._session  # local ref for thread safety
         if session and session.connected:
             session.send(data)
+        elif self._current_alias and not self._intentional_disconnect:
+            # Session dead — trigger reconnect
+            self._reconnect_count = 0
+            self._on_disconnected()
 
     def _on_resize(self, cols: int, rows: int):
         session = self._session  # local ref for thread safety