feat: multi-select and multi-drag on screen image area

[trmquang93]

Summary

• Shift/meta-click on a screen's image area toggles it in the current multi-selection
• Mousedown on an already multi-selected screen's image area starts a multi-drag
• Falls back to single-select otherwise; read-only mode preserved
• Sets cursor: default on the image wrapper

Test plan

• Shift-click image area adds/removes screen from selection
• Cmd-click behaves the same on macOS
• Dragging any member of a multi-selection moves the whole group
• Single click still selects a single screen
• Hotspot/comment/resize handle clicks are unaffected
• Read-only mode ignores shift/meta and multi-drag paths