fix(admin): render markdown preview from server-sanitized HTML

- MarkdownField posts the source to POST /markdown/preview when Preview
  opens and again 300 ms after a change while open; stale answers dropped
- the pane binds only data.html of a 2xx answer; a refusal is a text notice
- mlmarkdown previews the active locale and follows a locale switch
- .summer-markdown style kit restores headings, lists, code and tables
- vitest coverage, forms.md and rebuilt modules/boardwalk/dist

admin/src/styles/main.css

@@ -236,6 +236,116 @@
     outline-offset: 2px;
   }
 
+  /* Markdown preview: the server-rendered HTML of a markdown field. Preflight
+     strips heading, list and margin styles, so the pane restores them. */
+  .summer-markdown {
+    font-size: 14px;
+    line-height: 1.6;
+    color: var(--c-text);
+    overflow-wrap: anywhere;
+  }
+
+  .summer-markdown :where(h1, h2, h3, h4) {
+    margin: 0 0 8px;
+    line-height: 1.3;
+    font-weight: 600;
+    color: var(--c-text);
+  }
+
+  .summer-markdown :where(h1) {
+    font-size: 22px;
+  }
+
+  .summer-markdown :where(h2) {
+    font-size: 18px;
+  }
+
+  .summer-markdown :where(h3) {
+    font-size: 16px;
+  }
+
+  .summer-markdown :where(h4) {
+    font-size: 14px;
+  }
+
+  .summer-markdown :where(p, ul, ol, blockquote, pre, table) {
+    margin: 0 0 10px;
+  }
+
+  .summer-markdown :where(h1, h2, h3, h4, p, ul, ol, blockquote, pre, table):last-child {
+    margin-bottom: 0;
+  }
+
+  .summer-markdown :where(ul) {
+    list-style: disc;
+    padding-left: 22px;
+  }
+
+  .summer-markdown :where(ol) {
+    list-style: decimal;
+    padding-left: 22px;
+  }
+
+  .summer-markdown :where(code) {
+    font-family: var(--font-mono);
+    font-size: 13px;
+    background: var(--c-subtle);
+    border-radius: 4px;
+    padding: 1px 4px;
+  }
+
+  .summer-markdown :where(pre) {
+    font-family: var(--font-mono);
+    font-size: 13px;
+    background: var(--c-subtle);
+    border-radius: 8px;
+    padding: 10px 12px;
+    overflow-x: auto;
+    white-space: pre;
+  }
+
+  .summer-markdown :where(pre code) {
+    background: transparent;
+    padding: 0;
+  }
+
+  .summer-markdown :where(blockquote) {
+    border-left: 3px solid var(--c-border-strong);
+    padding-left: 12px;
+    color: var(--c-muted);
+  }
+
+  .summer-markdown :where(a) {
+    color: var(--c-text);
+    text-decoration: underline;
+  }
+
+  .summer-markdown :where(a):focus-visible {
+    outline: 3px solid var(--c-ring);
+    outline-offset: 2px;
+  }
+
+  .summer-markdown :where(hr) {
+    border: 0;
+    border-top: 1px solid var(--c-border);
+    margin: 12px 0;
+  }
+
+  .summer-markdown :where(table) {
+    border-collapse: collapse;
+  }
+
+  .summer-markdown :where(th, td) {
+    border: 1px solid var(--c-border);
+    padding: 4px 8px;
+    text-align: left;
+  }
+
+  .summer-markdown :where(img) {
+    max-width: 100%;
+    height: auto;
+  }
+
   .summer-stats {
     display: flex;
     flex-wrap: wrap;