Add copyright header support for board files (Fixes #7936)

[HammerGS]

Summary

• Adds CC BY-NC-SA 4.0 license header support when saving board files
• Header format matches existing mm-data board headers
• Year is dynamically set to current year when saving

Changes

GUIPreferences.java

• Added BOARD_SAVE_INCLUDE_LICENSE preference (default: true)
• Added getter/setter methods for the preference

Board.java

• Added LICENSE_HEADER constant with CC BY-NC-SA 4.0 text
• Added overloaded save(OutputStream, boolean) method to optionally include license
• Modified save(OutputStream) to check GUI preference

BoardsTagger.java

• Added runCopyrightHeaderUpdater() for batch processing all boards
• Added helper methods for recursive directory scanning and individual file updates

Test plan

• Ran the test,
• Made some boards and was able to add and tag them.

Fixes #7936

[Copilot]

Pull request overview

Adds optional CC BY-NC-SA 4.0 license headers to .board files when saving, with a GUI preference controlling inclusion and a utility method intended for batch-updating existing boards (Fixes #7936).

Changes:

• Introduces a new GUI preference (BOARD_SAVE_INCLUDE_LICENSE, default true) to control license-header inclusion.
• Extends Board.save(...) with an overload to include/exclude the license header and updates the default save path to consult the preference.
• Adds a batch updater method in BoardsTagger and a new unit test suite covering header inclusion and round-trip load/save.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
megamek/unittests/megamek/common/board/BoardLicenseHeaderTest.java	Adds tests validating header inclusion/exclusion, current-year insertion, and round-trip behavior.
megamek/src/megamek/utilities/BoardsTagger.java	Adds batch header updater + recursive scan helper for .board files.
megamek/src/megamek/common/board/Board.java	Adds the CC BY-NC-SA 4.0 header template and save overload; default save now uses GUI preference.
megamek/src/megamek/client/ui/clientGUI/GUIPreferences.java	Adds the new BOARD_SAVE_INCLUDE_LICENSE preference with default + getter/setter.

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