fix(frontend): la conversation survit au redemarrage agent et a l'aller-retour projet — #291 (ProjectsView/WebWorkspace conservent le pointeur viewerConversation mais ne rendent le viewer que si sa conversation appartient au projet actif: le projet intermediaire affiche sa propre surface sans fuite, le retour au projet source remonte la vue avec la meme cle project/conversation et relit le transcript persiste; useConversationThread ne vide plus le buffer au changement de conversation cible, un contenu deja visible reste affiche pendant relecture ou echec; l'erreur de lecture devient "Impossible de recharger la conversation." avec bouton Reessayer branchant refreshTail sans retirer les tours visibles; QA verte reconfirmee par Git: ProjectsView.ls7 scenario "rehydrates the same conversation" (restart agent, switch alpha->beta->alpha, deux readPage identiques sur {projectId, conversationId}), ConversationViewer + useConversationThread 19/19, npm run build, git diff --check; reserves hors perimetre preexistantes: 3 echecs historiques ProjectsView.ls7 (helper panneau Work/open conversation) + 1 echec WebDesktopWorkspace (libelle anglais Project context); validation runtime AppImage/live restante; aucun push)

frontend/src/features/conversations/ConversationViewer.test.tsx

@@ -4,7 +4,7 @@
  * states, scroll-up "load older" trigger, and the back button (`onClose`).
  */
 import { describe, it, expect, vi } from "vitest";
-import { render, screen, fireEvent } from "@testing-library/react";
+import { render, screen, fireEvent, within } from "@testing-library/react";
 import type { ReactNode } from "react";
 
 import { DIProvider } from "@/app/di";
@@ -256,7 +256,12 @@ describe("ConversationViewer", () => {
     };
     renderViewer({ conversation: failing });
     const alert = await screen.findByRole("alert");
-    expect(alert.textContent).toContain("lecture impossible");
+    expect(alert.textContent).toContain(
+      "Impossible de recharger la conversation.",
+    );
+    expect(
+      within(alert).getByRole("button", { name: "Réessayer" }),
+    ).toBeTruthy();
   });
 
   it("offers a 'load older' control when more turns exist and triggers loadOlder", async () => {

frontend/src/features/conversations/ConversationViewer.tsx

@@ -288,12 +288,20 @@ export function ConversationViewer({
 
       {/* ── Error ── */}
       {vm.error && (
-        <p
+        <div
           role="alert"
-          className="mx-4 mt-2 shrink-0 rounded-md border border-danger/40 bg-danger/10 px-3 py-2 text-sm text-danger"
+          className="mx-4 mt-2 flex shrink-0 items-center justify-between gap-3 rounded-md border border-danger/40 bg-danger/10 px-3 py-2 text-sm text-danger"
         >
-          {vm.error}
-        </p>
+          <span>Impossible de recharger la conversation.</span>
+          <Button
+            size="sm"
+            variant="ghost"
+            onClick={() => void vm.refreshTail()}
+            loading={vm.busy}
+          >
+            Réessayer
+          </Button>
+        </div>
       )}
 
       {/* ── Thread ── */}

frontend/src/features/conversations/useConversationThread.ts

@@ -151,10 +151,9 @@ export function useConversationThread(
 
   // (Re)open whenever the target conversation changes.
   useEffect(() => {
-    setBuffer([]);
     setHasMore(false);
     void open();
-  }, [open, setBuffer]);
+  }, [open]);
 
   return { turns, busy, error, hasMore, loadOlder, refreshTail };
 }