Add install folder action to Play menu

[UncleUncleRj]

Fixes #77.

What changed

• Replaced the Play control with a split button: Play still launches the selected game, while the arrow offers an install-folder action.
• Opens the selected game's bin directory with File Explorer on Windows, Finder on macOS, or the system file manager on Linux.
• Keeps the new action in sync with installation and download state, and aligns the Download button with the split control.
• Shows the complete executable path in the Play tooltip without Fluent's default 320-pixel cap.

Validation

• Parsed MainWindow.axaml as XML.
• Ran git diff --check.
• No build run; local Visual Studio builds are performed by the contributor.