Make the drop UI instant and window-wide

[ankitsejwal]

Follow-up on the sidebar rework. Two tweaks to the drag experience:

• The drop pad now shows the instant you bring a photo over the window — anywhere, not just when the cursor reaches the narrow sidebar. Detection moved to a small window-level hook (useWindowFileDrop), and a drop is accepted wherever it lands.
• The square pad is vertically centered in the rail now instead of clinging to the top, and the strip behind it blurs as before.

Also dropped the react-dropzone dependency since the hook covers both drag and the browse button (a plain hidden file input).