Fix HTML preview drag capture

[jsgrrchg]

Summary

• Add an internal drag iframe shield for HTML previews.
• Make HTML preview iframes temporarily inert while files or agent sessions are dragged from the sidebar.
• Cover file-tree, agent-sidebar, workspace-tab, and cancellation behavior in HtmlTabView tests.

Root Cause

HTML previews render inside an iframe. When an internal drag crosses that iframe, the frame can swallow the window-level move/up events that keep sidebar drags and multi-pane drop previews alive. The drag state can then remain stuck and leave the workspace drop system in a broken state.

Validation

• npm test -- HtmlTabView.test.tsx MultiPaneWorkspace.test.tsx AgentsSidebarPanel.test.tsx
• npx eslint src/features/editor/HtmlTabView.tsx src/features/editor/HtmlTabView.test.tsx src/features/editor/useInternalDragIframeShield.ts
• git diff --check

Fixes #165