feat: merge frontend and backend into one project

frontend/templates/_header.html

frontend/templates/_navigation.html

frontend/templates/error.html

frontend/templates/image_detail.html

frontend/templates/image_list.html

frontend/templates/index.html

frontend/templates/upload.html

frontend/util.go

go.mod

go.sum

shared/config.go

shared/task.go

shared/types.go

types.go