feat(frontend): système de plugins — runtime, menus, layouts custom (#43)

Lots F1-F4 : runtime de chargement/registre plugin, extension des menus
existants, panneau de gestion des plugins, types de layout custom
(sélecteur, fallback, cellule dédiée) branchés sur le port plugin.
Suite npm typecheck/test verte (947/947).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/layout/layout.ts

@@ -91,6 +91,10 @@ function mapNode(node: LayoutNode, f: (n: LayoutNode) => LayoutNode): LayoutNode
     case "leaf":
       rebuilt = node;
       break;
+    case "customPluginLayout":
+      // No children to recurse into — same leaf-like treatment as "leaf".
+      rebuilt = node;
+      break;
     case "split":
       rebuilt = {
         type: "split",
@@ -319,6 +323,52 @@ export function droppedSessions(
   return out;
 }
 
+/**
+ * Locally patches a `customPluginLayout` node's opaque `state` (#43, F4).
+ *
+ * Client-side only: the backend has no `LayoutOperation` variant to persist
+ * plugin layout state yet (carnet v2 §3.5 — no backend refonte expected for
+ * F4), so this does NOT call `LayoutGateway.mutateLayout`. It's the same
+ * "real, in-session, not yet cross-restart-persisted" contract every plugin
+ * component's `setState` gets: the tree re-renders with the new state
+ * immediately, but a reload re-fetches the last **persisted** value from the
+ * backend. Returns `tree` unchanged if no such node is found.
+ */
+export function setCustomPluginLayoutState(
+  tree: LayoutTree,
+  nodeId: string,
+  state: unknown,
+): LayoutTree {
+  return {
+    root: mapNode(tree.root, (n) => {
+      if (n.type === "customPluginLayout" && n.node.id === nodeId) {
+        return { type: "customPluginLayout", node: { ...n.node, state } };
+      }
+      return n;
+    }),
+  };
+}
+
+/**
+ * Locally replaces a `customPluginLayout` node with a blank terminal leaf of
+ * the same id (#43, F4 "Choisir un autre layout" fallback action). Same
+ * client-side-only contract as {@link setCustomPluginLayoutState} — no
+ * backend operation exists to persist the node-kind change yet.
+ */
+export function replaceCustomPluginLayoutWithTerminal(
+  tree: LayoutTree,
+  nodeId: string,
+): LayoutTree {
+  return {
+    root: mapNode(tree.root, (n) => {
+      if (n.type === "customPluginLayout" && n.node.id === nodeId) {
+        return { type: "leaf", node: { id: nodeId } };
+      }
+      return n;
+    }),
+  };
+}
+
 /** Convenience: builds a `split` operation splitting `target` in `direction`. */
 export function splitOp(target: string, direction: Direction): LayoutOperation {
   return {