fix(template): show tag display names instead of their slugs

[culfin]

The server-rendered templates print a tag's slug where the React interface
prints its display name:

<a href="{{$.baseURL}}/tags/{{.SlugName}}" class="badge-tag …">
  <span class="">{{.SlugName}}</span>
</a>

For Latin-script tags this is a matter of case — binokularsehen against
Binokularsehen. For everything else it is worse: Answer transliterates slugs
(hence mozillazg-go-unidecode among the dependencies), so a tag named 眼镜
renders as yan-jing on every server-rendered page.

Two audiences are affected: search engines, which see nothing else, and readers,
until the interface takes over.

Changed in four templates — question.html, question-detail.html,
tag-detail.html (the heading and the list) and tags.html. Only the text;
every href goes on using the slug.

Note

display_name is not null default in entity.Tag, so a tag with an empty
display name would now render as an empty badge. The API requires the field when
a tag is created, so this should not occur — but if you would rather have a
fallback, {{if .DisplayName}}…{{else}}{{.SlugName}}{{end}} is a one-line
change and I am happy to add it.

🤖 Generated with Claude Code

[culfin]

Folded into #1592 together with the other three defects in ui/template/ — one pull request is easier to review than four, and none of them is more than a line or two. Nothing is dropped, and the new one also targets dev as requested in #1572. Closing to keep the queue readable.