Feature: Add Upload component

It would be useful to have a reusable Upload component for handling file and folder uploads in desktop applications.

The component could support both traditional file selection and drag-and-drop interactions, providing a consistent way to handle file imports across applications.

Proposed features

• Click to open a native file picker
• Drag and drop files
• Support multiple files
• Optional folder selection
• Configurable accepted file types
• Disabled state
• Drag-over state
• Callback when files are selected or dropped

A basic usage could look like:

Upload::new()
    .accept(["png", "jpg", "zip"])
    .multiple(true)
    .on_change(...)
    .child(...)

Or provide a default dropzone-style UI:

┌──────────────────────────────────┐
│                                  │
│      Drop files here             │
│      or click to browse          │
│                                  │
└──────────────────────────────────┘

Use cases

This would be useful for common desktop application scenarios such as:

• Importing files or folders
• Importing models or assets
• Adding images
• Importing configuration files
• Drag-and-drop workflows

Implementation

The component could build on existing file picker and drag-and-drop capabilities where possible, while leaving the exact API and implementation details to gpui-kit.

The goal is to provide a reusable abstraction for file import interactions rather than requiring each application to implement the same behavior independently.