Make `add line comment` buttons focusable (#25894) (#25896)

Backport #25894 by @sebastian-sauer

Use a real button and add an aria-label.
Additionally, show the button whenever it is focused.
See https://codeberg.org/forgejo/forgejo/issues/998 for explanation.

Our handling of this button is now equal to that of GitHub.
Nothing has changed visually.

Co-authored-by: sebastian-sauer <sauer.sebastian@gmail.com>

options/locale/locale_en-US.ini

 diff.load = Load Diff
 diff.generated = generated
 diff.vendored = vendored
+diff.comment.add_line_comment = Add line comment
 diff.comment.placeholder = Leave a comment
 diff.comment.markdown_info = Styling with markdown is supported.
 diff.comment.add_single_comment = Add single comment

templates/repo/diff/section_split.tmpl

 					<td class="lines-type-marker lines-type-marker-old del-code"><span class="gt-mono" data-type-marker="{{$line.GetLineTypeMarker}}"></span></td>
 					<td class="lines-code lines-code-old del-code">{{/*
 						*/}}{{if and $.root.SignedUserID $.root.PageIsPullFiles}}{{/*
-							*/}}<a class="ui primary button add-code-comment add-code-comment-left{{if (not $line.CanComment)}} invisible{{end}}" data-side="left" data-idx="{{$line.LeftIdx}}">{{/*
+							*/}}<button type="button" aria-label="{{$.root.locale.Tr "repo.diff.comment.add_line_comment"}}" class="ui primary button add-code-comment add-code-comment-left{{if (not $line.CanComment)}} invisible{{end}}" data-side="left" data-idx="{{$line.LeftIdx}}">{{/*
 								*/}}{{svg "octicon-plus"}}{{/*
-							*/}}</a>{{/*
+							*/}}</button>{{/*
 						*/}}{{end}}{{/*
 						*/}}{{if $line.LeftIdx}}{{/*
 							*/}}{{template "repo/diff/section_code" dict "diff" $leftDiff "locale" $.root.locale}}{{/*
 					<td class="lines-type-marker lines-type-marker-new add-code">{{if $match.RightIdx}}<span class="gt-mono" data-type-marker="{{$match.GetLineTypeMarker}}"></span>{{end}}</td>
 					<td class="lines-code lines-code-new add-code">{{/*
 						*/}}{{if and $.root.SignedUserID $.root.PageIsPullFiles}}{{/*
-							*/}}<a class="ui primary button add-code-comment add-code-comment-right{{if (not $match.CanComment)}} invisible{{end}}" data-side="right" data-idx="{{$match.RightIdx}}">{{/*
+							*/}}<button type="button" aria-label="{{$.root.locale.Tr "repo.diff.comment.add_line_comment"}}" class="ui primary button add-code-comment add-code-comment-right{{if (not $match.CanComment)}} invisible{{end}}" data-side="right" data-idx="{{$match.RightIdx}}">{{/*
 								*/}}{{svg "octicon-plus"}}{{/*
-							*/}}</a>{{/*
+							*/}}</button>{{/*
 						*/}}{{end}}{{/*
 						*/}}{{if $match.RightIdx}}{{/*
 							*/}}{{template "repo/diff/section_code" dict "diff" $rightDiff "locale" $.root.locale}}{{/*
 					<td class="lines-type-marker lines-type-marker-old">{{if $line.LeftIdx}}<span class="gt-mono" data-type-marker="{{$line.GetLineTypeMarker}}"></span>{{end}}</td>
 					<td class="lines-code lines-code-old">{{/*
 						*/}}{{if and $.root.SignedUserID $.root.PageIsPullFiles (not (eq .GetType 2))}}{{/*
-							*/}}<a class="ui primary button add-code-comment add-code-comment-left{{if (not $line.CanComment)}} invisible{{end}}" data-side="left" data-idx="{{$line.LeftIdx}}">{{/*
+							*/}}<button type="button" aria-label="{{$.root.locale.Tr "repo.diff.comment.add_line_comment"}}" class="ui primary button add-code-comment add-code-comment-left{{if (not $line.CanComment)}} invisible{{end}}" data-side="left" data-idx="{{$line.LeftIdx}}">{{/*
 								*/}}{{svg "octicon-plus"}}{{/*
-							*/}}</a>{{/*
+							*/}}</button>{{/*
 						*/}}{{end}}{{/*
 						*/}}{{if $line.LeftIdx}}{{/*
 							*/}}{{template "repo/diff/section_code" dict "diff" $inlineDiff "locale" $.root.locale}}{{/*
 					<td class="lines-type-marker lines-type-marker-new">{{if $line.RightIdx}}<span class="gt-mono" data-type-marker="{{$line.GetLineTypeMarker}}"></span>{{end}}</td>
 					<td class="lines-code lines-code-new">{{/*
 						*/}}{{if and $.root.SignedUserID $.root.PageIsPullFiles (not (eq .GetType 3))}}{{/*
-							*/}}<a class="ui primary button add-code-comment add-code-comment-right{{if (not $line.CanComment)}} invisible{{end}}" data-side="right" data-idx="{{$line.RightIdx}}">{{/*
+							*/}}<button type="button" aria-label="{{$.root.locale.Tr "repo.diff.comment.add_line_comment"}}" class="ui primary button add-code-comment add-code-comment-right{{if (not $line.CanComment)}} invisible{{end}}" data-side="right" data-idx="{{$line.RightIdx}}">{{/*
 								*/}}{{svg "octicon-plus"}}{{/*
-							*/}}</a>{{/*
+							*/}}</button>{{/*
 						*/}}{{end}}{{/*
 						*/}}{{if $line.RightIdx}}{{/*
 							*/}}{{template "repo/diff/section_code" dict "diff" $inlineDiff "locale" $.root.locale}}{{/*

templates/repo/diff/section_unified.tmpl

 			{{else}}
 				<td class="chroma lines-code{{if (not $line.RightIdx)}} lines-code-old{{end}}">{{/*
 					*/}}{{if and $.root.SignedUserID $.root.PageIsPullFiles}}{{/*
-						*/}}<a class="ui primary button add-code-comment add-code-comment-{{if $line.RightIdx}}right{{else}}left{{end}}{{if (not $line.CanComment)}} invisible{{end}}" data-side="{{if $line.RightIdx}}right{{else}}left{{end}}" data-idx="{{if $line.RightIdx}}{{$line.RightIdx}}{{else}}{{$line.LeftIdx}}{{end}}">{{/*
+						*/}}<button type="button" aria-label="{{$.root.locale.Tr "repo.diff.comment.add_line_comment"}}" class="ui primary button add-code-comment add-code-comment-{{if $line.RightIdx}}right{{else}}left{{end}}{{if (not $line.CanComment)}} invisible{{end}}" data-side="{{if $line.RightIdx}}right{{else}}left{{end}}" data-idx="{{if $line.RightIdx}}{{$line.RightIdx}}{{else}}{{$line.LeftIdx}}{{end}}">{{/*
 							*/}}{{svg "octicon-plus"}}{{/*
-						*/}}</a>{{/*
+						*/}}</button>{{/*
 					*/}}{{end}}{{/*
 					*/}}{{template "repo/diff/section_code" dict "diff" $inlineDiff "locale" $.root.locale}}{{/*
 				*/}}</td>

web_src/css/review.css

   opacity: 1;
 }
 
+.ui.button.add-code-comment:focus {
+  opacity: 1;
+}
+
 .repository .diff-file-box .code-diff .add-comment-left,
 .repository .diff-file-box .code-diff .add-comment-right,
 .repository .diff-file-box .code-diff .add-code-comment .add-comment-left,