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

@@ -2366,6 +2366,7 @@ diff.show_more = Show More
 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