Hi, I'm Product Manager at vocabla.com. I work on design, UI, UX on a daily basis. Redesigning is what I do almost all the time, so I'm fluent with Photoshop. Up to date, for over 1,5 year I've prepared more than 200 enhancements for Vocabla for web, mobile & tablet app. I always prepare structured PSD files organized in logical groups of layers with proper naming etc. I am familiar with responsive design, also experimenting with flat concepts. I'm familiar with brainstorming, providing ideas and solutions to common UI problems etc.
Please note: I'm not sure if by "mockup" you mean wireframe (prototype) or PSD design. "Mockup" is often used for both. Anyway, I can do both. Also, my bid is NOT adequate, as I don't know the scope of your web app. Can start ASAP and we can start with test-small project to check if we find our cooperation promising. I freelance during evenings in GMT+1 timezone. Best Regards, Kuba