Replace drag surfaces with drag handle, fix hover interactions

[alizme]

Summary

• Fix focus ring lag after reorder: Added #nodeVersion counter to EditorState so overlay measurements re-trigger when sibling/parent nodes change (not just the selected node)
• Replace large drag surfaces with drag handle: Removed the full-frame auto/absolute drag surface overlays that blocked hover and click interactions on child frames. Replaced with a small 16x16px drag handle icon at top-center of the selected frame
• Robust hover detection: Use document.elementsFromPoint to find content elements through overlays, extracted into updateHover() called from both pointermove and keydown/keyup so Cmd+hover instantly shows the parent focus ring without requiring cursor movement
• Cmd+hover parent promotion: When Cmd is held and cursor is over the selected node or any of its descendants, the hover ring targets the selected node's parent container frame

Test plan

• Select a frame in a layout, drag to reorder — verify focus ring moves instantly with the frame
• Select a parent container frame — verify children are hoverable and clickable
• Use the drag handle (blue dot icon above frame) to drag/reorder frames
• Hold Cmd while hovering a selected frame — verify parent's focus ring appears instantly
• Release Cmd — verify hover reverts to deepest frame under cursor
• Resize frames via edge/corner handles — verify resize still works
• Undo/redo operations — verify focus ring updates correctly

🤖 Generated with Claude Code