Fix Alt+Enter client crash

[nickalreadyinuse]

Toggles between fullscreen and windowed/borderless now.

[Copilot]

Pull request overview

This PR fixes a client crash that occurred when pressing Alt+Enter to toggle fullscreen mode. The crash was caused by DXGI's built-in Alt+Enter handler calling SetFullscreenState internally without the application's knowledge, which put the swap chain in an inconsistent state and caused the next Present() call to fail.

Changes:

• Disabled DXGI's built-in Alt+Enter handling using MakeWindowAssociation with DXGI_MWA_NO_ALT_ENTER
• Implemented custom Alt+Enter handling in the message handler to toggle between fullscreen and windowed modes
• Added renderer null check in the message handler for safety
• Fixed missing break statement after WM_ACTIVATEAPP case

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
game_patch/graphics/d3d11/gr_d3d11_hooks.cpp	Added custom Alt+Enter handler in WM_SYSKEYDOWN, added renderer null check, and fixed missing break statement
game_patch/graphics/d3d11/gr_d3d11.cpp	Disabled DXGI's built-in Alt+Enter handling via MakeWindowAssociation
docs/CHANGELOG.md	Added changelog entry documenting the fix

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[is-this-c]

The reason Present fails is because Alpine Faction uses DXGI's flip presentation model which requires ResizeBuffers after mode transitions. Your comments should include this detail.