Checkpoint: headless JSON architecture documented + cleanup (Stufe 1+2)

Restore point before Stufe 3 (central resolver-service refactoring).

Includes this session's work:
- Success Story (usecase) rebuild: new fields/tabs, UsecaseSerializer,
  thin List/Show renderers, MM relations, inline content elements.
- vitec_columns content block (two-column layout with per-item content)
  incl. unified header section; special-cased JSON resolution.
- Container per-column flex (items[].config align/justify) + gap on parent.
- Unified header section across CEs/plugins/containers; header fields in JSON.
- ISO-style architecture spec: Documentation/Headless-JSON-Architecture.md.
- Cleanup: removed local scratch + verified .bak backups.
- Fixes: B-6 (undefined thumbnail variable in Downloadcardcollection image
  resolver), B-7 (unsatisfiable legacy CType OR branch in Downloadcard/Datasheets).

Rollback: git reset --hard snapshot-2026-07-09-pre-stufe3

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/vitec/Resources/Public/Css/backend-container-collapse.css

@@ -0,0 +1,22 @@
+/* =============================================================
+   VITEC — container collapse/expand in the Page module
+   ============================================================= */
+
+/* Hide the container body (nested child columns) when collapsed. */
+.t3-page-ce.vitec-ce-collapsed > .t3-page-ce-element > .t3-page-ce-body {
+    display: none;
+}
+
+/* Optional: dim/round the collapsed container header a touch. */
+.t3-page-ce.vitec-ce-collapsed > .t3-page-ce-element > .t3-page-ce-header {
+    border-bottom: 1px dashed var(--typo3-component-border-color, #ccc);
+}
+
+/* Chevron rotates when collapsed. */
+.vitec-collapse-toggle typo3-backend-icon {
+    display: inline-flex;
+    transition: transform 0.15s ease-in-out;
+}
+.t3-page-ce.vitec-ce-collapsed .vitec-collapse-toggle typo3-backend-icon {
+    transform: rotate(180deg);
+}