Remove QgsProject::instance() references from QgsMapViewsManager (QEP 423)

[JanCaha]

Description

Remove QgsProject::instance() from QgsMapViewsManager. The project is passed as required constructor argument and actually stored as parent of QObject. So it can be obtained from there.

AI tool usage

• AI tool(s) (Copilot, Claude, or something similar) supported my development of this PR.

[github-actions]

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(Built from commit 6ff0465)

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