Add drag & drop upload to QR Code → Text

[Copilot]

The QR Code → Text decoder only supported click-to-upload via a plain button. This adds a proper drag-and-drop zone as the primary upload interaction.

Changes

• Drop zone UI — Replaces the upload button with a styled dashed-border zone; supports both drag-and-drop and click-to-open-picker
• Drag-over highlight — isDragOver state toggles border-primary/bg-primary/5 on enter/leave
• onDragLeave flicker fix — Guards against spurious leave events when cursor moves over child elements:

  onDragLeave={(e) => {
    if (!e.currentTarget.contains(e.relatedTarget as Node)) {
      setIsDragOver(false)
    }
  }}

• Shared decodeFile callback — Extracts image-reading/jsQR logic into a useCallback consumed by both the drop handler and the file <input> onChange