fix(frontend): persistance immédiate de l'historique des prompts à chaque envoi — #323 (QA verte)

Symptôme: l'historique du composer CLI custom (flèche haut) n'était
écrit dans localStorage qu'via un effet sur l'état, sans garantie au
moment de l'envoi — une fermeture brutale/crash perdait la dernière
entrée.

- writeComposerHistory retourne désormais le succès de l'écriture; à
  chaque envoi, l'entrée est ajoutée et écrite AVANT l'invocation de
  l'agent (source de vérité par ref + état, réinit de l'index de
  navigation et du brouillon d'historique)
- l'effet d'écriture global est supprimé: l'historique n'est plus
  réécrit en boucle, seulement aux envois (et lectures au changement de
  scope projet/layout/node)
- si l'écriture échoue, l'envoi part quand même et un avertissement
  discret « Historique des prompts non sauvegardé. » (role=status)
  s'affiche au-dessus du composer
- tests: persistance vérifiée au moment de l'appel sendAgentChat,
  envoi conservé + avertissement affiché quand localStorage échoue

QA verte sur fix/batch-322-323-session-restore-prompt-history. Aucun push.

frontend/src/features/agents/CustomAgentChatView.test.tsx

@@ -864,6 +864,66 @@ describe("CustomAgentChatView", () => {
     await waitFor(() => expect(composer.value).toBe("remember me"));
   });
 
+  it("persists an accepted text prompt before invoking the agent", async () => {
+    const historyKey = "idea.custom-chat-history.project-1.layout-1.node-1.agent-1";
+    let historyAtSend: string[] = [];
+    const agent = {
+      launchAgentChat: vi.fn(),
+      reattachAgentChat: vi.fn(async (sessionId: string) => ({ sessionId, scrollback: [] })),
+      sendAgentChat: vi.fn(
+        async (_sessionId: string, _prompt: string, onChunk: (chunk: ReplyChunk) => void) => {
+          historyAtSend = JSON.parse(window.localStorage.getItem(historyKey) ?? "[]");
+          onChunk({ kind: "final", content: "done" });
+        },
+      ),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+    render(
+      <DIProvider gateways={{ agent, system: { pickFile: vi.fn(async () => null) } } as unknown as Gateways}>
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          layoutId="layout-1"
+          nodeId="node-1"
+          sessionId="chat-session-1"
+          conversationId="conversation-1"
+          onSessionId={vi.fn()}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    const composer = await screen.findByLabelText(/message CLI custom/);
+    fireEvent.change(composer, { target: { value: "persist before send" } });
+    fireEvent.click(screen.getByRole("button", { name: "Envoyer" }));
+
+    await waitFor(() => expect(agent.sendAgentChat).toHaveBeenCalledOnce());
+    expect(historyAtSend).toEqual(["persist before send"]);
+  });
+
+  it("keeps sending when prompt-history persistence fails and reports it discreetly", async () => {
+    const setItem = vi.spyOn(Storage.prototype, "setItem").mockImplementation(() => {
+      throw new Error("storage unavailable");
+    });
+    try {
+      const agent = renderHistoryChat();
+      const composer = await screen.findByLabelText(/message CLI custom/);
+      fireEvent.change(composer, { target: { value: "still send" } });
+      fireEvent.click(screen.getByRole("button", { name: "Envoyer" }));
+
+      await waitFor(() => expect(agent.sendAgentChat).toHaveBeenCalledOnce());
+      expect(screen.getByRole("status").textContent).toContain(
+        "Historique des prompts non sauvegardé.",
+      );
+    } finally {
+      setItem.mockRestore();
+    }
+  });
+
   it("repaints attachment labels from user prompt scrollback", async () => {
     const agent = {
       launchAgentChat: vi.fn(),

frontend/src/features/agents/CustomAgentChatView.tsx

@@ -472,15 +472,16 @@ function readComposerHistory(storageKey: string): string[] {
   }
 }
 
-function writeComposerHistory(storageKey: string, history: string[]): void {
-  if (typeof window === "undefined") return;
+function writeComposerHistory(storageKey: string, history: string[]): boolean {
+  if (typeof window === "undefined") return true;
   try {
     window.localStorage.setItem(
       storageKey,
       JSON.stringify(history.slice(-COMPOSER_HISTORY_LIMIT)),
     );
+    return true;
   } catch {
-    /* local history only */
+    return false;
   }
 }
 
@@ -1151,6 +1152,8 @@ export function CustomAgentChatView({
   const [composerHistory, setComposerHistory] = useState<string[]>(() =>
     readComposerHistory(composerHistoryKey),
   );
+  const composerHistoryRef = useRef(composerHistory);
+  const [composerHistoryWarning, setComposerHistoryWarning] = useState<string | null>(null);
   const [composerHistoryIndex, setComposerHistoryIndex] =
     useState<number | null>(null);
   const displayTurns = turns;
@@ -1304,15 +1307,13 @@ export function CustomAgentChatView({
 
   useEffect(() => {
     const restored = readComposerHistory(composerHistoryKey);
+    composerHistoryRef.current = restored;
     setComposerHistory(restored);
+    setComposerHistoryWarning(null);
     setComposerHistoryIndex(null);
     draftBeforeHistoryNavigationRef.current = "";
   }, [composerHistoryKey]);
 
-  useEffect(() => {
-    writeComposerHistory(composerHistoryKey, composerHistory);
-  }, [composerHistory, composerHistoryKey]);
-
   // Attachments can be large, so keep draft bytes only in this renderer's
   // bounded lifetime instead of serialising base64 into localStorage. The
   // project/layout/node scope mirrors the persisted text draft contract.
@@ -2246,6 +2247,17 @@ export function CustomAgentChatView({
   async function send() {
     const text = draft.trim();
     if (!canSend || !agent.sendAgentChat) return;
+    if (text) {
+      const nextHistory = appendComposerHistoryEntry(composerHistoryRef.current, text);
+      composerHistoryRef.current = nextHistory;
+      const persisted = writeComposerHistory(composerHistoryKey, nextHistory);
+      setComposerHistory(nextHistory);
+      setComposerHistoryWarning(
+        persisted ? null : "Historique des prompts non sauvegardé.",
+      );
+      setComposerHistoryIndex(null);
+      draftBeforeHistoryNavigationRef.current = "";
+    }
     const slashInvocation = slashCommandInvocation(text);
     if (slashInvocation) {
       await executeSlashCommand(slashInvocation);
@@ -2263,11 +2275,6 @@ export function CustomAgentChatView({
       markProjectCliInteractionFinished(projectId, interactionId);
       activeInteractionIdRef.current = null;
     };
-    if (text) {
-      setComposerHistory((prev) => appendComposerHistoryEntry(prev, text));
-      setComposerHistoryIndex(null);
-      draftBeforeHistoryNavigationRef.current = "";
-    }
     setDraft("");
     setAttachments([]);
     imageTokenSeqRef.current = 0;
@@ -2705,6 +2712,11 @@ export function CustomAgentChatView({
             })}
           </div>
         )}
+        {composerHistoryWarning && (
+          <p role="status" className="mb-1 text-[11px] text-warning">
+            {composerHistoryWarning}
+          </p>
+        )}
         <div className="flex min-w-0 items-end gap-2">
           <div className="relative flex min-w-0 flex-1">
             <textarea