Enhance file size validation and limits

Currently, the GUI accepts APK and IPA files without any size restrictions, which can lead to several problems:

- Large files can cause memory issues during upload
- Excessive storage usage in MinIO
- Potential timeout issues during processing
- Poor user experience with no upload progress indication

Technical Requirements:

- Backend Validation (Python/FastAPI):
- Add configurable file size limit (default: 100MB, configurable via environment variable)
- Implement file size check in the /upload endpoint before MinIO upload
- Add proper error handling with descriptive messages
- Log file size information for monitoring

Frontend Enhancement (HTML/JavaScript):

- Display file size in the upload interface
- Add client-side size validation before upload
- Show upload progress bar for large files
- Display remaining upload time estimate

Configuration:

- Add MAX_FILE_SIZE_MB environment variable
- Update Helm charts to include the new environment variable
- Document the configuration option

Acceptance Criteria:

[ ] File size validation implemented in backend (configurable via env var)
[ ] Clear error messages for oversized files ("File too large. Maximum size: 100MB")
[ ] Frontend displays file size and upload progress
[ ] Client-side validation prevents unnecessary uploads
[ ] Documentation updated with size limits and configuration
[ ] Helm charts updated with new environment variable
[ ] Tested with files of various sizes (small, medium, large, oversized)

Files to Modify:

- gui/webapp/main.py - Backend validation
- gui/webapp/templates/index.html - Frontend enhancements
- gui/values.yaml - Helm configuration
- gui/templates/webapp-deployment.yaml - Environment variables
- documentation/markdown/gui-setup.md - Documentation

GLHF :)