feat: gate document upload until documentation API is available

[abrar-deakin]

Summary

• Added file type validation for PDF, JPG, and PNG documents.
• Added a 10MB file size limit.
• Guard document uploads continue to use local storage while the Documentation API upload endpoint is unavailable.
• Added clear feedback to inform the guard that the document was saved locally and the API upload is not available yet.
• Keeps the feature safely gated until the backend Documentation API endpoint is confirmed.

Testing

• Tested document selection and upload in the Guard App.
• Confirmed valid PDF upload is saved and displayed in Uploaded Documents.
• Confirmed the local storage/API unavailable feedback message.
• Confirmed document deletion still works.