fix(explorer): support multi-file drag and folder content drops

[ThisIs-Developer]

What changed

• Preserve a multi-file Explorer selection when any selected file starts a drag.
• Move the full selected set with one drop and retain the legacy single-file drag payload for compatibility.
• Add a stacked drag preview with a clear selected-file count.
• Treat expanded folder and workspace contents as drop surfaces, including small empty-folder targets.
• Resolve nested drop surfaces to the deepest matching folder so parent workspaces do not steal the drop.
• Refresh the generated Neutralino desktop resources and document the interaction.

Why

The Explorer already supported multi-selection and bulk overflow actions, but drag start stored only one document ID. Folder drops were also bound only to the folder-name row, so dropping over files inside an expanded folder fell through to another location.

The updated behavior follows familiar Finder and VS Code conventions: dragging one item from a selection operates on the complete selection, and an open folder behaves as a destination across its visible contents.

User impact

Users can now select several Markdown files and drag them together. They can drop on a folder name, on an existing file inside an expanded folder, or into an expanded empty folder. Hover expansion, Explorer auto-scroll, keyboard/touch Move to…, and bulk more-options actions remain available.

Validation

• node --check script.js
• node --check desktop-app/resources/js/script.js
• npm run build
• Explorer Playwright suite: 13 passed, including an exact temporary regression for multi-file drag onto expanded folder contents
• Dark-theme Files tree Playwright QA: passed
• node desktop-app/prepare.js with offline asset validation
• git diff --check

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Project	Deployment	Actions	Updated (UTC)
markdown-viwer	Ready	Preview	Aug 2, 2026 8:57pm