feat(admin): sortable form-help blocks with two-panel UI

- Migration 005: add sort_order column to form_help_blocks
- Database: getAllFormHelpBlocks orders by sort_order; new reorderFormHelpBlocks()
- actions/form-help-reorder.php: HTMX POST handler, CSRF-validated, 204 response
- templates/admin/contenus.php: replace flat table with two-panel layout
  - Left: SortableJS 1.15.2 + htmx drag-and-drop ordered block cards
  - Right: static form structure reference showing fieldsets and their inputs
- admin.css: .fhb-* styles for layout, cards, ghost/chosen/drag states, anchors
- schema.sql: updated form_help_blocks DDL with sort_order column

app/public/assets/css/admin.css

@@ -1589,3 +1589,236 @@
     color: var(--text-secondary);
     font-style: italic;
 }
+
+/* ═══════════════════════════════════════════════════════════════════════════
+   Form Help Blocks — drag-and-drop builder (contenus.php)
+   ═══════════════════════════════════════════════════════════════════════════ */
+
+.fhb-hint {
+    color: var(--text-secondary);
+    font-size: var(--step--1);
+    margin-bottom: var(--space-m);
+}
+
+.fhb-layout {
+    display: grid;
+    grid-template-columns: 1fr 1fr;
+    gap: var(--space-m);
+    align-items: start;
+    margin-top: var(--space-m);
+}
+
+@media (max-width: 800px) {
+    .fhb-layout {
+        grid-template-columns: 1fr;
+    }
+}
+
+/* ── Panels ─────────────────────────────────────────────────────────────── */
+
+.fhb-sortable-panel,
+.fhb-form-preview-panel {
+    border: 1px solid var(--border-primary);
+    border-radius: 6px;
+    padding: var(--space-s);
+    background: var(--bg-secondary);
+}
+
+.fhb-panel-title {
+    font-size: var(--step-0);
+    font-weight: 600;
+    margin: 0 0 var(--space-3xs) 0;
+    letter-spacing: 0.03em;
+}
+
+.fhb-panel-desc {
+    font-size: var(--step--2);
+    color: var(--text-secondary);
+    margin: 0 0 var(--space-xs) 0;
+}
+
+/* ── Saving indicator ─────────────────────────────────────────────────────── */
+
+.fhb-saving {
+    display: none;
+    align-items: center;
+    gap: var(--space-2xs);
+    font-size: var(--step--1);
+    color: var(--accent-primary);
+    padding: var(--space-2xs) 0;
+}
+
+.fhb-saving.htmx-request {
+    display: flex;
+}
+
+/* ── Draggable block cards ─────────────────────────────────────────────────── */
+
+.fhb-sortable {
+    display: flex;
+    flex-direction: column;
+    gap: var(--space-2xs);
+    padding: 0;
+    margin: 0;
+}
+
+.fhb-block-card {
+    display: flex;
+    align-items: center;
+    gap: var(--space-xs);
+    background: var(--bg-primary);
+    border: 1px solid var(--border-primary);
+    border-left: 4px solid var(--accent-primary);
+    border-radius: 4px;
+    padding: var(--space-2xs) var(--space-xs);
+    cursor: default;
+    transition: box-shadow 0.15s, border-color 0.15s;
+}
+
+.fhb-block-card:hover {
+    box-shadow: 0 2px 8px rgba(0,0,0,0.08);
+    border-color: var(--accent-primary);
+}
+
+.fhb-drag-handle {
+    font-size: 1.2em;
+    color: var(--text-tertiary);
+    cursor: grab;
+    flex-shrink: 0;
+    line-height: 1;
+    user-select: none;
+    padding: 2px 4px;
+}
+
+.fhb-drag-handle:active {
+    cursor: grabbing;
+}
+
+.fhb-block-info {
+    flex: 1;
+    min-width: 0;
+    display: flex;
+    flex-direction: column;
+    gap: 2px;
+}
+
+.fhb-block-label {
+    font-size: var(--step--1);
+    font-weight: 500;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+
+.fhb-block-preview {
+    font-size: var(--step--2);
+    color: var(--text-secondary);
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+
+.fhb-block-empty {
+    font-size: var(--step--2);
+    color: var(--text-tertiary);
+    font-style: italic;
+}
+
+.fhb-edit-btn {
+    flex-shrink: 0;
+    font-size: var(--step--2) !important;
+    padding: 2px var(--space-xs) !important;
+}
+
+/* ── SortableJS state classes ─────────────────────────────────────────────── */
+
+.fhb-ghost {
+    opacity: 0.35;
+    background: var(--accent-muted);
+    border-color: var(--accent-primary);
+}
+
+.fhb-chosen {
+    box-shadow: 0 4px 16px rgba(149, 87, 181, 0.25);
+    border-color: var(--accent-primary);
+}
+
+.fhb-dragging {
+    opacity: 0.9;
+    box-shadow: 0 8px 24px rgba(0,0,0,0.15);
+}
+
+/* ── Form structure preview (right panel) ─────────────────────────────────── */
+
+.fhb-form-preview {
+    display: flex;
+    flex-direction: column;
+    gap: var(--space-2xs);
+}
+
+.fhb-fieldset-preview {
+    border: 1px solid var(--border-secondary);
+    border-radius: 4px;
+    padding: var(--space-xs);
+    background: var(--bg-primary);
+}
+
+.fhb-fieldset-legend {
+    font-size: var(--step--1);
+    font-weight: 600;
+    color: var(--text-primary);
+    margin-bottom: var(--space-3xs);
+    padding-bottom: var(--space-3xs);
+    border-bottom: 1px solid var(--border-primary);
+}
+
+.fhb-fieldset-inputs {
+    margin: 0;
+    padding: 0 0 0 var(--space-s);
+    list-style: disc;
+}
+
+.fhb-fieldset-inputs li {
+    font-size: var(--step--2);
+    color: var(--text-secondary);
+    line-height: 1.6;
+}
+
+.fhb-anchor {
+    display: flex;
+    align-items: center;
+    gap: var(--space-2xs);
+    border-radius: 4px;
+    padding: var(--space-3xs) var(--space-xs);
+    font-size: var(--step--2);
+    border: 1px dashed var(--border-primary);
+    background: transparent;
+}
+
+.fhb-anchor--filled {
+    border-color: var(--accent-primary);
+    background: var(--accent-muted);
+    color: var(--accent-secondary);
+}
+
+.fhb-anchor--empty {
+    color: var(--text-tertiary);
+}
+
+.fhb-anchor-icon {
+    flex-shrink: 0;
+    font-style: normal;
+}
+
+.fhb-anchor-label {
+    flex: 1;
+}
+
+.fhb-anchor-pos {
+    font-size: var(--step--2);
+    font-weight: 600;
+    color: var(--accent-primary);
+    background: var(--accent-muted);
+    border-radius: 2px;
+    padding: 0 4px;
+}