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/plugins/PluginLayoutSelectorSection.tsx

@@ -0,0 +1,73 @@
+/**
+ * `PluginLayoutSelectorSection` — the "Layouts plugins" section building block
+ * (ticket #43, F4, carnet §10: "Section `Layouts plugins` dans le sélecteur").
+ *
+ * Lists every plugin layout contribution currently loaded (disabled/missing
+ * plugins never appear — carnet §1.3: only `enabled` plugins are loaded, so
+ * there is nothing to filter here beyond what the registry already omits).
+ *
+ * Presentational only; not yet mounted in a layout-creation flow. The
+ * existing "layout" concept in this codebase (`LayoutTabs`, `LayoutKind`) is
+ * a whole-tab kind (`"terminal" | "gitGraph"`) picked via a fixed two-item
+ * dropdown, backed by a `create(name, kind)` Tauri command that only knows
+ * those two kinds. Wiring an actual "create a plugin layout" action needs a
+ * backend `LayoutKind`/`create_layout` extension (carnet §10 flags F4 as
+ * "DevFrontend + DevBackend si ajustement DTO layout") — this component is
+ * the frontend half, ready to drop into that flow once the DTO lands; see the
+ * F4 delivery report's open point.
+ */
+
+import type { PluginLayoutContribution } from "@/domain";
+import type { PluginRuntimeRegistry } from "@/plugins/runtime";
+
+export interface PluginLayoutChoice {
+  pluginId: string;
+  pluginDisplayName: string;
+  layout: PluginLayoutContribution;
+}
+
+export function listPluginLayoutChoices(registry: PluginRuntimeRegistry): PluginLayoutChoice[] {
+  return registry
+    .layoutContributions()
+    .map(({ pluginId, pluginDisplayName, layout }) => ({ pluginId, pluginDisplayName, layout }))
+    .sort(
+      (a, b) =>
+        (a.layout.order ?? 0) - (b.layout.order ?? 0) ||
+        a.pluginDisplayName.localeCompare(b.pluginDisplayName) ||
+        a.layout.label.localeCompare(b.layout.label),
+    );
+}
+
+interface PluginLayoutSelectorSectionProps {
+  registry: PluginRuntimeRegistry;
+  onSelect: (choice: PluginLayoutChoice) => void;
+}
+
+export function PluginLayoutSelectorSection({
+  registry,
+  onSelect,
+}: PluginLayoutSelectorSectionProps) {
+  const choices = listPluginLayoutChoices(registry);
+  if (choices.length === 0) return null;
+
+  return (
+    <div role="group" aria-label="Layouts plugins">
+      <p className="px-3 pt-2 text-[0.65rem] font-semibold uppercase tracking-wide text-faint">
+        Layouts plugins
+      </p>
+      {choices.map((choice) => (
+        <button
+          key={`${choice.pluginId}:${choice.layout.type}`}
+          type="button"
+          role="menuitem"
+          aria-label={`create ${choice.layout.label} layout`}
+          onClick={() => onSelect(choice)}
+          className="flex w-full items-center justify-between gap-2 px-3 py-2 text-left text-sm text-content hover:bg-raised"
+        >
+          <span>{choice.layout.label}</span>
+          <span className="text-xs text-faint">{choice.pluginDisplayName}</span>
+        </button>
+      ))}
+    </div>
+  );
+}