Migrate from Qt5 to Qt6

[mchamberland]

Summary

• Port eb_gui from Qt5 to Qt6 so it builds cleanly with modern Qt (widgets + charts).
• Replace removed APIs (QRegExp/QRegExpValidator, QString::SkipEmptyParts, QSignalMapper::mapped, mouse/wheel event helpers, QString/char/int conversions, font weights).
• Update the qmake project for C++17 and a plain ../eb_gui executable, and refresh the README install notes.
• Align macOS gui_location with the non-bundle layout by removing the old .app/Contents/MacOS upward path adjustment.
• Enable QT_DISABLE_DEPRECATED_BEFORE=0x060000 so remaining pre-Qt6 deprecated APIs fail at compile time.

Test plan

• With Qt 6 installed: cd source && qmake6 && make
• Confirm ../eb_gui links against Qt 6 frameworks/libraries and launches
• On macOS, confirm configuration.txt, database/MAR_defaults.txt, and database/metric_defaults.txt are found relative to the executable directory (not via a .app path traversal)
• Smoke-test DICOM load, phantom/source tabs, and dose charts/isodose interactions
• On dual Qt5/Qt6 systems, verify qmake6 (not Qt5 qmake) is used