add optional Show on All Desktops mode

[zhouzhou-1921]

Summary

• add a Show on All Desktops toggle under the existing Display menu
• persist the setting with UserDefaults and apply the same all-Spaces window behavior to the character window, popover, and thinking/completion bubble
• update controller visibility gating so the option takes effect immediately for existing windows without changing the default behavior

Notes

• this keeps the current behavior as the default
• display pinning still determines the geometry anchor screen
• existing windows are updated immediately when the toggle changes

Validation

• lightweight Swift tests pass
• xcodebuild -project "lil-agents.xcodeproj" -scheme "LilAgents" -configuration Debug build
• manual local validation confirmed the cross-Space effect works

Closes #41