Media: Prevent drag and drop uploads in media states that don't support uploading

[nhrrob]

Stops the media modal's "Drop files to upload" overlay and drag-and-drop uploads in states that aren't for uploading (crop, edit image, image/audio/video details, insert from URL), via a new uploader: false state attribute.

Testing instructions

1. Go to Settings > General > Site Icon, choose an image larger than 512×512 and click "Set as Site Icon" to reach the Crop step.
2. Drag a file from your computer over the modal.
3. Before: the "Drop files to upload" overlay appears and dropping uploads the file. After: no overlay and nothing is uploaded.
4. Close the modal, open it again and drag a file onto the Media Library tab: it still uploads as before.
5. Optionally repeat steps 1–3 in Appearance > Customize > Site Identity > Site Icon.

Trac ticket: https://core.trac.wordpress.org/ticket/47130

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Drafting the change and verifying it with Playwright; reviewed and tested by me.

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