Adds a Submit Job modal to the Matrix 3D workshop UI with comprehensive
client-side validation:
- Submit button disabled when title is empty or inputs exceed max length
- Character count displayed for title (0/200) and description (0/2000)
- Warning shown when description exceeds 1800 characters
- Inline error messages (red text) for validation failures
- Validation runs on input, blur, and submit attempt
- Success state shown after form submission
- Responsive design with mobile support
- Keyboard accessible (Escape to close, Enter to submit)
Fixes#754
