Enable page-wide WASM drag and drop

[espmaniac]

What changed

• make the entire page accept dropped files instead of limiting drops to the upload card
• show a full-viewport overlay while files are dragged over the site
• accept only file drags, leaving text and link dragging untouched
• use nested drag-enter tracking so the overlay does not flicker over child elements
• preserve the upload card, file picker, and existing multi-file task launch path

Why

A page-wide target is faster and more forgiving: users can release a WASM app anywhere without aiming at one specific element.

Validation

• node --check main.js
• git diff --check
• page-wide drag/drop event smoke test covering nested enters/leaves, file drops, copy feedback, and ignored text drags
• exact public commit preview reached Runtime: OK
• verified the fixed overlay matches the viewport, its stylesheet loaded, and the page has no horizontal overflow
• GitHub Actions Build browser runtime run #66 completed successfully