admin.css: replace .admin-main, .admin-page-title, .admin-table, .admin-fieldset with semantic selectors

Replace four presentational class names in admin.css with structural selectors
that target native HTML elements already present in every admin template:

  .admin-main           → .admin-body main
  .admin-page-title     → .admin-body main > h1
  .admin-table          → .admin-body table
  .admin-fieldset       → .admin-body fieldset
  .admin-fieldset-legend → .admin-body legend

Also migrate the .admin-main > section / h2 / dl / dt / dd block to
.admin-body main > section so the thanks-page section styles survive.

Add .admin-body main > table { margin-top: 1.5rem } to absorb the inline
style="margin-top:1.5rem" that was on tags.php's <table class="admin-table">.

All 10 affected admin templates updated (add, edit, account, index, import,
pages, pages-edit, tags, system, thanks) — class attributes removed where
the element alone is now the selector.  Zero visual changes.

public/assets/css/admin.css

@@ -87,14 +87,14 @@
 }
 
 /* Main content area */
-.admin-main {
+.admin-body main {
   flex: 1;
   padding: 2.5rem 2rem 4rem;
   max-width: 1100px;
   width: 100%;
 }
 
-.admin-page-title {
+.admin-body main > h1 {
   font-size: 1.8rem;
   font-weight: 600;
   letter-spacing: 0.08em;
@@ -375,13 +375,18 @@
 }
 
 /* ---- Table ---- */
-.admin-table {
+.admin-body table {
   width: 100%;
   border-collapse: collapse;
   font-size: 0.88rem;
 }
 
-.admin-table th {
+/* margin-top for the tags table (only table not immediately after stats/filters) */
+.admin-body main > table {
+  margin-top: 1.5rem;
+}
+
+.admin-body table th {
   text-align: left;
   font-size: 0.75rem;
   letter-spacing: 0.08em;
@@ -393,23 +398,23 @@
   white-space: nowrap;
 }
 
-.admin-table td {
+.admin-body table td {
   padding: 0.65rem 0.75rem;
   border-bottom: 1px solid var(--admin-border);
   color: var(--admin-text);
   vertical-align: top;
 }
 
-.admin-table tr:hover td {
+.admin-body table tr:hover td {
   background: var(--admin-bg-alt);
 }
 
-.admin-table .thesis-title {
+.admin-body table .thesis-title {
   font-weight: 500;
   color: var(--admin-text);
 }
 
-.admin-table .thesis-subtitle {
+.admin-body table .thesis-subtitle {
   font-size: 0.82rem;
   color: var(--admin-text-muted);
   font-style: italic;
@@ -521,33 +526,33 @@
 }
 
 /* Thesis info sections (thanks page) */
-.admin-main > section {
+.admin-body main > section {
   border: 1px solid var(--admin-border);
   border-radius: 6px;
   padding: 1.5rem;
   margin-bottom: 1.5rem;
 }
 
-.admin-main > section h2 {
+.admin-body main > section h2 {
   margin: 0 0 1rem;
   font-size: 1.2rem;
   border-bottom: 1px solid var(--admin-border);
   padding-bottom: 0.5rem;
 }
 
-.admin-main > section dl {
+.admin-body main > section dl {
   display: grid;
   grid-template-columns: 180px 1fr;
   gap: 0.4rem 1rem;
 }
 
-.admin-main > section dt {
+.admin-body main > section dt {
   font-weight: 600;
   font-size: 0.88rem;
   color: var(--admin-text-muted);
 }
 
-.admin-main > section dd {
+.admin-body main > section dd {
   margin: 0;
   font-size: 0.9rem;
 }
@@ -622,7 +627,7 @@
 }
 
 /* Jury fieldset */
-.admin-fieldset {
+.admin-body fieldset {
   border: 1px solid #333;
   border-radius: 4px;
   padding: 1rem 1.25rem;
@@ -630,7 +635,7 @@
   background: rgba(255, 255, 255, 0.02);
 }
 
-.admin-fieldset-legend {
+.admin-body legend {
   font-size: 0.82rem;
   font-weight: 600;
   letter-spacing: 0.04em;