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/LayoutGrid.tsx

@@ -38,6 +38,7 @@ import {
   TargetAnnouncementsOverlay,
   useTargetAnnouncements,
 } from "@/features/announcements";
+import { PluginLayoutCellView } from "@/features/plugins";
 import {
   modelServerOverlayText,
   describeModelServerDownload,
@@ -79,9 +80,21 @@ interface LayoutGridProps {
   layoutId?: string;
   /** Opens the read-only canonical transcript for a conversation. */
   onOpenConversation?: (conversationId: string) => void;
+  /**
+   * Navigates to `Paramètres > Plugins` (#43, F4) — the "Ouvrir Plugins" action
+   * of a plugin layout's unavailable fallback. Threaded down to
+   * `PluginLayoutCellView` the same way `onOpenConversation` is.
+   */
+  onOpenPluginsSettings?: () => void;
 }
 
-export function LayoutGrid({ projectId, cwd, layoutId, onOpenConversation }: LayoutGridProps) {
+export function LayoutGrid({
+  projectId,
+  cwd,
+  layoutId,
+  onOpenConversation,
+  onOpenPluginsSettings,
+}: LayoutGridProps) {
   const vm = useLayout(projectId, layoutId);
   const work = useProjectWorkState(projectId);
 
@@ -117,6 +130,7 @@ export function LayoutGrid({ projectId, cwd, layoutId, onOpenConversation }: Lay
         workState={work.state}
         refreshWorkState={work.refresh}
         onOpenConversation={onOpenConversation}
+        onOpenPluginsSettings={onOpenPluginsSettings}
       />
     </div>
   );
@@ -132,6 +146,7 @@ interface NodeViewProps {
   workState: ProjectWorkState | null;
   refreshWorkState: () => Promise<void>;
   onOpenConversation?: (conversationId: string) => void;
+  onOpenPluginsSettings?: () => void;
 }
 
 function NodeView({
@@ -143,8 +158,24 @@ function NodeView({
   workState,
   refreshWorkState,
   onOpenConversation,
+  onOpenPluginsSettings,
 }: NodeViewProps) {
   switch (node.type) {
+    case "customPluginLayout":
+      // A true top-level `LayoutNode` variant (#43, F4, carnet v2 §3) — a
+      // plugin layout occupies a slot in the tree at the same level as a
+      // terminal leaf, split or grid. Rendered separately from `LeafView`
+      // (which owns a lot of terminal-only concerns — write-portal,
+      // model-server overlay, agent dropdown — none of which apply here).
+      return (
+        <PluginLayoutCellView
+          projectId={projectId}
+          cell={node.node}
+          onStateChange={(nextState) => vm.setPluginLayoutState(node.node.id, nextState)}
+          onOpenPlugins={() => onOpenPluginsSettings?.()}
+          onChooseAnotherLayout={() => vm.replacePluginLayoutWithTerminal(node.node.id)}
+        />
+      );
     case "leaf":
       return (
         <LeafView
@@ -172,6 +203,7 @@ function NodeView({
           workState={workState}
           refreshWorkState={refreshWorkState}
           onOpenConversation={onOpenConversation}
+          onOpenPluginsSettings={onOpenPluginsSettings}
         />
       );
     case "grid":
@@ -184,6 +216,7 @@ function NodeView({
           workState={workState}
           refreshWorkState={refreshWorkState}
           onOpenConversation={onOpenConversation}
+          onOpenPluginsSettings={onOpenPluginsSettings}
         />
       );
   }
@@ -1153,6 +1186,7 @@ interface SplitViewProps {
   workState: ProjectWorkState | null;
   refreshWorkState: () => Promise<void>;
   onOpenConversation?: (conversationId: string) => void;
+  onOpenPluginsSettings?: () => void;
 }
 
 function SplitView({
@@ -1163,6 +1197,7 @@ function SplitView({
   workState,
   refreshWorkState,
   onOpenConversation,
+  onOpenPluginsSettings,
 }: SplitViewProps) {
   const isRow = split.direction === "row";
   const baseWeights = split.children.map((c) => c.weight);
@@ -1207,6 +1242,7 @@ function SplitView({
               workState={workState}
               refreshWorkState={refreshWorkState}
               onOpenConversation={onOpenConversation}
+              onOpenPluginsSettings={onOpenPluginsSettings}
               parentSplit={{
                 container: split.id,
                 index: i,
@@ -1301,6 +1337,7 @@ interface GridViewProps {
   workState: ProjectWorkState | null;
   refreshWorkState: () => Promise<void>;
   onOpenConversation?: (conversationId: string) => void;
+  onOpenPluginsSettings?: () => void;
 }
 
 function GridView({
@@ -1311,6 +1348,7 @@ function GridView({
   workState,
   refreshWorkState,
   onOpenConversation,
+  onOpenPluginsSettings,
 }: GridViewProps) {
   const cols = normalizeWeights(grid.colWeights)
     .map((p) => `${p}fr`)
@@ -1351,6 +1389,7 @@ function GridView({
             workState={workState}
             refreshWorkState={refreshWorkState}
             onOpenConversation={onOpenConversation}
+            onOpenPluginsSettings={onOpenPluginsSettings}
           />
         </div>
       ))}