[Application] Improve event drag validation feedback

[95x8x9]

Summary

• allow scenario events to keep following the cursor while dragging and validate placement when dropped
• show a warning and rollback when a block, hazard, or guidance event is dropped on an invalid target
• stabilize the blocked-door invalid preview so the translucent marker stays visually clear while dragging

Related Issue

• Closes Task-시나리오 이벤트 드래그 검증 피드백 개선 #254

Area

• Engine
• Domain
• Application
• Docs
• Build
• Analysis
• Chore

Architecture Check

• I kept the dependency direction application -> domain -> engine.
• I did not add Qt UI code to src/domain.
• I did not add domain or application dependencies to src/engine.
• I used src/ as the include root.

Verification

• cmake --preset windows-debug
• cmake --build --preset build-debug
• ctest --preset test-debug
• Not run (reason below)

Risks / Follow-up

• event drag behavior now flows through shared preview and rollback handling in ScenarioCanvasWidget, so future interaction changes should retest drag, selection, and context-menu behavior together