Implement standardized native dialog opt-in and unified file logic

[google-labs-jules]

Context & Rationale

Researchers on macOS currently face a suboptimal experience where file dialogs do not support platform-standard features like the Favorites sidebar or Tag filtering. This is caused by a known conflict between the IPython event loop and native Qt dialogs, which can cause panels to close prematurely in certain configurations.

This PR introduces a unified utility for handling file dialogs and provides an experimental opt-in for native OS dialogs. By making this feature strictly opt-in, we satisfy the needs of power users who require a native macOS experience while maintaining stability for the general user base.

Key Changes

1. Unified Dialog Logic

• Refactored _select_file in src/eegprep/functions/guifunc/qt.py to utilize the _file_dialog_kwargs helper.
• Consolidated all menu-driven file selection logic to ensure "Open Data" and "Save As" actions behave consistently across the application.

2. Global Configuration & Persistence

• Added option_native_dialogs to the EEGOptions dataclass in src/eegprep/functions/adminfunc/eeg_options.py.
• Set the default value to 0 (disabled) to ensure application stability on first launch. This setting is persisted in the user's global configuration.

3. Settings UI Enhancement

• Upgraded the preferences window in src/eegprep/functions/guifunc/menu_actions.py from a simple message box to a comprehensive QDialog.
• Implemented an Experimental Features section in the settings UI containing a "Use Native OS Dialogs" toggle.

Design Decisions

• Stable-by-Default: To prevent system regressions, the application remains on non-native Qt dialogs unless the user explicitly opts in. This protects against the known IPython pulsed event loop issues.
• Dynamic Dispatching: Menu action handlers now query EEG_OPTIONS directly when spawning file panels. This allows users to toggle the native dialog setting and see the change reflected immediately without restarting the application.
• Scope Limitation: This change specifically targets file selection panels (Open/Save). Modal alerts and other UI elements remain unchanged to minimize the surface area for potential event loop conflicts.

Success Metrics & Validation

• Verified that the "Native Dialogs" toggle persists between application restarts.
• Confirmed that when the toggle is disabled, the application uses the reliable (non-native) Qt dialog style.
• Validated that toggling the preference does not interfere with the IPython console's ability to process input.