Port GUI to Qt6/qtpy>=2.4 and PyQt6: replace deprecated Qt enums/APIs and update deps

[axelvonkamp]

Motivation

• Ensure compatibility with newer Qt/PyQt/qtpy stacks (Qt6 / qtpy>=2.4 / PyQt6) by replacing deprecated Qt API usage and enum names.
• Unify usage of explicit enum types and roles (cursor shapes, color roles, key/mouse/modifier enums, message box icons, header/size policies, etc.) to avoid runtime attribute errors under Qt6.
• Update packaging metadata to require newer Qt-related packages consistent with the API changes.

Description

• Replaced many legacy Qt.* enum usages with the Qt6-style names such as Qt.GlobalColor.*, Qt.CursorShape.*, Qt.Key.*, Qt.MouseButton.*, Qt.KeyboardModifier.*, Qt.ApplicationAttribute, Qt.ColorRole/QPalette.ColorRole, QPalette.ColorGroup, QFrame.Shape/QFrame.Shadow, and related enum classes across multiple GUI modules.
• Updated various Qt container/role/mode APIs to use namespaced enum types from PyQt6/qtpy (e.g. QHeaderView.ResizeMode, QAbstractItemView.SelectionMode, QTableWidget.setEditTriggers with EditTrigger, QCompleter.CompletionMode, QMenu.exec replacement for exec_, QDialog.exec for exec_).
• Replaced QMessageBox icon and button constants with QMessageBox.Icon.* and QMessageBox.StandardButton.* where appropriate.
• Made cursor, drag and drop and cursor restore calls use the new CursorShape enums and switched several exec_() calls to exec() for Qt6 compatibility.
• Adjusted qt high DPI attribute setting to use Qt.ApplicationAttribute lookups to remain safe under Qt5/Qt6.
• Updated project dependency files: environment.yml and pyproject.toml to require qtpy>=2.4, pyqt6>=6.8 / pyqt6-webengine, and qtconsole>=5.6 (and matching webengine entries) to match the API changes.
• Changes are mechanical and applied across many GUI modules (e.g., application.py, appdata.py, core_gui.py, numerous files under gui_elements/, utils.py, and more) to modernize enum uses and API calls.

Testing

• No automated tests were added to this PR and no test suite was executed as part of the rollout; the changes are mechanical API replacements intended to be a compatibility layer for Qt6/qtpy>=2.4 and require running the project's existing unit/UI test suite and a manual UI smoke test on a Qt6-enabled environment to verify runtime behavior.

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Codex Task