fix: align templates with wireframe styling

- Fix article header tag borders: replace broken border-current/20
  (Tailwind can't apply opacity to currentColor) with per-tag border
  colour classes via new get_tag_border_css filter
- Add calendar icon before article date in article header
- Add clock icon before read time in article header and home featured
- Match article card footer to wireframe (remove extra min-read span)
- Add rounded-md to code block matching wireframe

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

templates/blog/blocks/code_block.html

@@ -1,5 +1,5 @@
 {% load wagtailcore_tags %}
-<div class="my-8 overflow-hidden bg-[#0d1117] border border-zinc-800 shadow-xl">
+<div class="my-8 rounded-md overflow-hidden bg-[#0d1117] border border-zinc-800 shadow-xl">
   <div class="flex items-center justify-between px-4 py-2 bg-[#161b22] border-b border-zinc-800">
     <div class="flex gap-2">
       <div class="w-3 h-3 rounded-full bg-red-500"></div>