feat(agent): UI hot-swap de profil (A2) — commande Tauri + sélecteur + dialog

- Tauri : commande change_agent_profile + ChangeAgentProfileRequestDto/Dto
  (camelCase, relaunchedSession omis si None), câblage state.rs par composition.
- Front : gateway changeAgentProfile (adapters Tauri+mock), sélecteur de profil
  par agent, dialog de confirmation FR (« Changer le moteur abandonne l'historique
  de conversation… »), refresh sur event agentProfileChanged.

Tests : app-tauri dto 5/5 ; vitest 314/314 (mapping, payload, dialog gating,
libellé FR, refresh). 0 régression.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/agents/AgentsPanel.tsx

@@ -127,6 +127,31 @@ export function AgentsPanel({ projectId, projectRoot = "" }: AgentsPanelProps) {
     {},
   );
 
+  /**
+   * Pending profile change awaiting confirmation: the target agent + the chosen
+   * profile id. `null` when no dialog is open. Switching the engine abandons the
+   * conversation history, so it is gated behind an explicit confirmation.
+   */
+  const [pendingProfileChange, setPendingProfileChange] = useState<{
+    agentId: string;
+    profileId: string;
+  } | null>(null);
+
+  async function confirmProfileChange() {
+    if (!pendingProfileChange) return;
+    const { agentId, profileId } = pendingProfileChange;
+    setPendingProfileChange(null);
+    // Use the running agent terminal's size when available; fall back to a sane
+    // default (the backend only needs these for a possible hot relaunch).
+    const relaunched = await vm.changeAgentProfile(agentId, profileId, 24, 80);
+    // On a hot relaunch the backend mints a fresh session; rebind the panel's
+    // terminal to it and drop any persisted conversation id for this cell (the
+    // history is gone — the front reflects the swap for the current session).
+    if (relaunched && agentId === activeAgentId) {
+      setAgentSessions((prev) => ({ ...prev, [agentId]: relaunched.sessionId }));
+    }
+  }
+
   const canCreate = newName.trim().length > 0 && !vm.busy;
 
   async function handleCreate(e: React.FormEvent) {
@@ -332,6 +357,35 @@ export function AgentsPanel({ projectId, projectRoot = "" }: AgentsPanelProps) {
                   </button>
 
                   <div className="flex items-center gap-1.5 shrink-0">
+                    {/* Profile hot-swap selector (Chantier A). Changing the
+                        engine abandons the conversation history → confirmation. */}
+                    {vm.profiles.length > 0 && (
+                      <select
+                        aria-label={`profile for ${a.name}`}
+                        value={a.profileId}
+                        disabled={vm.busy}
+                        onChange={(e) => {
+                          const profileId = e.target.value;
+                          if (profileId && profileId !== a.profileId) {
+                            setPendingProfileChange({ agentId: a.id, profileId });
+                          }
+                        }}
+                        className={cn(
+                          "h-8 rounded-md bg-raised px-2 text-xs text-content",
+                          "border border-border outline-none transition-colors",
+                          "focus:border-primary disabled:cursor-not-allowed disabled:opacity-50",
+                        )}
+                      >
+                        {vm.profiles.every((p) => p.id !== a.profileId) && (
+                          <option value={a.profileId}>{a.profileId}</option>
+                        )}
+                        {vm.profiles.map((p) => (
+                          <option key={p.id} value={p.id}>
+                            {p.name}
+                          </option>
+                        ))}
+                      </select>
+                    )}
                     {agentDrift && (
                       <Button
                         size="sm"
@@ -517,6 +571,45 @@ export function AgentsPanel({ projectId, projectRoot = "" }: AgentsPanelProps) {
           </div>
         </div>
       )}
+
+      {/* ── Profile-change confirmation dialog ── */}
+      {pendingProfileChange && (
+        <div
+          role="dialog"
+          aria-modal="true"
+          aria-label="Confirm profile change"
+          className="fixed inset-0 z-50 flex items-center justify-center bg-black/55 p-4"
+        >
+          <div className="w-full max-w-sm rounded-lg border border-border bg-surface p-5 shadow-xl">
+            <h4 className="text-sm font-semibold text-content">
+              Changer le moteur IA
+            </h4>
+            <p className="mt-2 text-sm text-muted">
+              Changer le moteur abandonne l'historique de conversation (le
+              contexte et la mémoire sont conservés). Continuer&nbsp;?
+            </p>
+            <div className="mt-4 flex items-center justify-end gap-2">
+              <Button
+                variant="ghost"
+                aria-label="cancel profile change"
+                disabled={vm.busy}
+                onClick={() => setPendingProfileChange(null)}
+              >
+                Annuler
+              </Button>
+              <Button
+                variant="primary"
+                aria-label="confirm profile change"
+                loading={vm.busy}
+                disabled={vm.busy}
+                onClick={() => void confirmProfileChange()}
+              >
+                Continuer
+              </Button>
+            </div>
+          </div>
+        </div>
+      )}
     </Panel>
   );
 }