Update documentation and CLI behavior for GPU management

- Clarified the behavior of `Ctrl+C` and `Ctrl+D` in the README and other documentation, specifying that `Ctrl+C` only stops the tunnel while keeping the GPU active, and `Ctrl+D` stops the GPU while preserving disk data.
- Enhanced the CLI documentation to reflect these changes, ensuring users understand the implications of these commands during GPU operations.
- Improved the handling of data bindings and remounting logic in the codebase to prevent issues with empty model tabs in the UI.
- Added tests to validate the new command behaviors and ensure proper documentation alignment.

src/gpu_rent/local_watchdog.py

@@ -271,7 +271,7 @@ def install_watchdog(
     log(
         f"local-watchdog установлен ({platform}): тик каждые {interval} мин. "
         f"Grace {grace_seconds() // 60} мин после смерти процесса туннеля → stop. "
-        f"Ctrl+C на туннеле GPU не гасит. "
+        f"Ctrl+C на туннеле GPU не гасит; Ctrl+D — stop. "
         "При SELECTEL_API_TOKEN — toast каждые BALANCE_NOTIFY_STEP_RUB ₽ (дефолт 200)."
     )
     return marker