Dropzone styling improvements (#15291) (#15374)

* Dropzone styling improvements

- Move all dropzone styles to separate file
- Fix white background in arc-green
- Fix rendering of non-square images and previews

* increase thumbnail quality, set contain in js, replace blur effect with opacity

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/js/index.js

@@ -997,6 +997,9 @@ async function initRepository() {
             dictFileTooBig: $dropzone.data('file-too-big'),
             dictRemoveFile: $dropzone.data('remove-file'),
             timeout: 0,
+            thumbnailMethod: 'contain',
+            thumbnailWidth: 480,
+            thumbnailHeight: 480,
             init() {
               this.on('success', (file, data) => {
                 filenameDict[file.name] = {
@@ -2601,6 +2604,9 @@ $(document).ready(async () => {
       dictFileTooBig: $dropzone.data('file-too-big'),
       dictRemoveFile: $dropzone.data('remove-file'),
       timeout: 0,
+      thumbnailMethod: 'contain',
+      thumbnailWidth: 480,
+      thumbnailHeight: 480,
       init() {
         this.on('success', (file, data) => {
           filenameDict[file.name] = data.uuid;