Add ability to cancel ongoing deployments in Dev Center

[jelveh]

Currently, once a deployment starts in Dev Center, users have no way to cancel it even if they realize they selected the wrong files or the deployment is taking too long. This can be frustrating, especially with large deployments.

All the relevant code resides in dev-center.js

Technical Requirements:

1. Add a cancel button to the deployment progress interface:

   • Replace or augment the current "Deploying (X%)" message with a cancellable interface
   • Add a clear visual indicator that the deployment can be cancelled (e.g., "✕" button)

2. Implement proper cancellation handling:

   • Cancel any ongoing file uploads/transfers
   • Clean up any partially uploaded files in the app directory
   • Reset the drop area to its initial state
   • Show feedback that the deployment was cancelled

3. Consider states where cancellation might not be possible:

   • If deployment is near completion
   • During critical operations that shouldn't be interrupted
   • Add appropriate UI feedback for such cases

User Experience Flow:

1. User starts deployment
2. During "Deploying (X%)", user can click cancel
3. Show brief "Cancelling..." message
4. Clean up partial deployment
5. Reset UI to pre-deployment state

Expected Behavior:

• The cancel button should be clearly visible during deployment
• Cancellation should be responsive and immediate
• User should receive clear feedback about the cancellation
• The app should return to a clean state after cancellation