feat(ui): add BlockEditor component with block types, slash menu, and drag-and-drop

- add BlockEditor orchestrator with controlled block list and keyboard navigation
- add Block client component with contentEditable sync, drag handles, and markdown shortcuts
- add SlashMenu for inserting block types via `/` command
- add blockRegistry and block type definitions (paragraph, heading, bullet list, numbered list, quote, code, divider)
- add caret and id utility helpers
- export BlockEditor from shared components index
- add BlockEditor demo to admin devkit ComponentsPage
- add README documenting usage and architecture

src/shared/components/BlockEditor/utils/caret.js

@@ -0,0 +1,47 @@
+// Helpers de gestion du caret pour les contentEditable mono-bloc.
+// On reste sur du texte brut au MVP : un seul Text node enfant (ou aucun).
+
+export function getCaretOffset(el) {
+  const sel = typeof window !== 'undefined' ? window.getSelection() : null;
+  if (!sel || sel.rangeCount === 0) return 0;
+  const range = sel.getRangeAt(0);
+  if (!el.contains(range.startContainer)) return 0;
+  const pre = range.cloneRange();
+  pre.selectNodeContents(el);
+  pre.setEnd(range.startContainer, range.startOffset);
+  return pre.toString().length;
+}
+
+export function setCaretOffset(el, offset) {
+  if (!el) return;
+  el.focus();
+  const sel = window.getSelection();
+  if (!sel) return;
+  const range = document.createRange();
+  const text = el.firstChild;
+  if (!text) {
+    range.setStart(el, 0);
+    range.collapse(true);
+  } else {
+    const max = text.textContent?.length ?? 0;
+    const pos = Math.max(0, Math.min(offset, max));
+    range.setStart(text, pos);
+    range.collapse(true);
+  }
+  sel.removeAllRanges();
+  sel.addRange(range);
+}
+
+export function focusEnd(el) {
+  if (!el) return;
+  const len = (el.textContent ?? '').length;
+  setCaretOffset(el, len);
+}
+
+export function isCaretAtStart(el) {
+  return getCaretOffset(el) === 0;
+}
+
+export function isCaretAtEnd(el) {
+  return getCaretOffset(el) === (el.textContent ?? '').length;
+}

src/shared/components/BlockEditor/utils/ids.js

@@ -0,0 +1,8 @@
+// Génère un ID stable pour un bloc. randomUUID est dispo dans tous les navigateurs
+// modernes côté client ; en SSR on retombe sur un fallback simple.
+export function newBlockId() {
+  if (typeof crypto !== 'undefined' && typeof crypto.randomUUID === 'function') {
+    return crypto.randomUUID();
+  }
+  return `b_${Date.now().toString(36)}_${Math.random().toString(36).slice(2, 10)}`;
+}