cleanup: remove dead code and reorganize file structure

[Rubyherp]

Dead code removed:

• DockPanel: iconSize, containerView, addDragHandle() (drag handle was added via DockDragHandleRepresentable in SwiftUI instead)
• AppDelegate: stale panel.iconSize assignments, commented-out menu item
• DockDragHandleView: commented-out calculation
• DockState: unused items property (items read from @binding instead)

Reorganized:

• Models/: AppInfo (from Views/), DockState (from root/)
• Views/: DockDragHandleRepresentable (from Windows/), ScreenRectReader (extracted from TooltipManager.swift)
• Windows/: DockResizeHandleView (from Views/)
• Utilities/: new folder, moved Extension.swift from root/

Summary

• What changed?
• Why was this change needed?

Testing

• swift test
• Manual verification done (if relevant)

Checklist

• Scope is focused and minimal
• Docs updated (if needed)
• No secrets or private data added