Renamed PageView to BasePage throughout the codebase for clarity and consistency. Updated all imports, type hints, documentation, and test references to use BasePage, PageMediaData, and PageResizeEvent from the new module. Adjusted documentation and mkdocs navigation to reflect the change.

Close #5532

Summary by Sourcery

Rename PageView to BasePage for clarity and consistency and update all related references across the codebase.

Enhancements:

• Rename the PageView class and its imports, all exports, and subclass relationships to BasePage
• Update type hints and isinstance checks to use BasePage in base_control and control_event modules
• Adjust flet.init to export BasePage instead of PageView

Documentation:

• Rename and add documentation file controls/basepage.md and remove controls/pageview.md
• Update mkdocs navigation to reference BasePage instead of PageView