feat: merge frontend and backend into one project

frontend/assets/js/page_upload.js

frontend/assets/sass/_navigation.sass

frontend/assets/sass/_page_image_detail.sass

frontend/assets/sass/_page_image_list.sass

frontend/assets/sass/_page_upload.sass

frontend/assets/sass/_pagination.sass

frontend/assets/sass/fonts.sass

frontend/assets/sass/style.sass

frontend/errors.go

frontend/main.go

frontend/package-lock.json

frontend/package.json

frontend/page_album_detail.go

frontend/page_album_list.go

frontend/page_image_detail.go

frontend/page_image_list.go

frontend/page_index.go

frontend/page_upload.go

frontend/templates/_base.html

frontend/templates/_footer.html