fix(webapp): smooth out the first GitHub deployment in onboarding

Improve the first GitHub deployment experience: Deploy now explains when a branch doesn't exist on GitHub, a harmless first-build cache message no longer shows as an error, the deployment panel stays on screen after the first deploy finishes, the empty development Tasks page uses the new setup layout, and the deployment setup screen is vertically centered.

Mono-RevId: 07d4623e6fbe912906e1976f513f962c5ec42aa6