fix(agent-manager): reset stale body styles when webview regains focus (#530)

Kobalte modal dropdowns set pointer-events:none and overflow:hidden on
document.body. When the user opens the setup script editor from the
settings dropdown, VS Code steals focus before the dropdown cleanup
runs, leaving the body permanently unclickable. Clear these stale
styles in the existing window focus handler so the UI recovers when
the user returns to the agent manager tab.