Add drag-and-drop trace import

[ding-swj]

Body:
Support dragging a .json trace file directly into the AgentScope UI.

Scope:

• Add a drop zone or global drag target
• Reuse the existing trace validation logic
• Show the same error banner for invalid traces
• Keep the current file picker import working

Acceptance criteria:

• Dragging examples/auth-fix.trace.json loads the trace
• Invalid JSON shows a clear error
• npm run lint and npm run build pass