fix(template): repair avatar images in the server-rendered question list

[culfin]

The avatar in ui/template/question.html is built as:

<img src="{{$.baseURL}}/users/{{.Operator.Avatar}}" width="24px" height="24px"
     class="rounded-circle me-1" alt="shuai" data-processed="true">

.Operator.Avatar already holds the full avatar reference, so prefixing it with
/users/ yields an address that cannot resolve. Every question in the
server-rendered list renders a broken image — ten of them on a default page.
This is also the only template that adds the prefix; the four other places that
show an avatar use the value directly:

file	line
ui/template/question-detail.html	src="{{.detail.UserInfo.Avatar}}"
ui/template/question-detail.html	src="{{.UserInfo.Avatar}}" (twice)
ui/template/homepage.html	src="{{.userinfo.Avatar}}"

Two further defects sit in the same tag:

• alt="shuai" is placeholder text left over from a copied DOM. A screen
  reader announces it once per avatar. It now carries the display name of the
  person who asked.
• width="24px" / height="24px" are invalid — HTML dimension attributes
  take a number without a unit. Browsers discard them and fall back to the
  intrinsic size of the image, which makes every row of the list taller than the
  interface renders it.

How it was found

Building a site that shows the server-rendered pages to visitors rather than
only to crawlers. All three defects are invisible as long as the React
interface replaces the markup a moment later.

Note

Where a user has no avatar set, .Operator.Avatar is empty and src="" remains
— the interface substitutes default-avatar.svg in that case, the templates do
not. That is a separate question and left untouched here to keep this change to
the defect described.

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