feat(frontend): sessions list/create + steering + resume + selectors

- NavSessions: left nav listing sessions via listSessions (TanStack Query),
  clicking resumes via resumeSession and sets active; active item highlighted
- NewSessionDialog: ShadCn dialog with question textarea, workspace native
  select (from listWorkspaces), graceful model field (dropdown when models
  list is non-empty, free-text input when empty per §9 degradation),
  optional thinking/provider fields; try/catch surfaces errors inline
- SteerInput: text input + button POSTs to postSteer, clears on send,
  supports Enter key; only shown when a session is active
- WorkflowBar: renders 8 phases (F1..F8) with data-active highlighting
  driven by useSessionStore.currentPhase (new store field + setPhase action)
- AppShell: wires all four components; right artifact sidebar now collapsible
  via a toggle button (useState sidebarOpen); replaces old "Nuova domanda"
  button with NewSessionDialog trigger
- f1-loop.test: updated to drive through the new dialog flow (open → fill
  question → submit → wait for SSE → respond)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/shell/NavSessions.tsx

@@ -0,0 +1,44 @@
+// frontend/src/shell/NavSessions.tsx
+import { useQuery } from "@tanstack/react-query";
+import { listSessions } from "../api/sessions";
+import { resumeSession } from "../api/sessions";
+import type { SessionSummary } from "../api/types";
+
+interface Props {
+  activeSessionId: string | null;
+  onSessionSelect: (id: string) => void;
+}
+
+export function NavSessions({ activeSessionId, onSessionSelect }: Props) {
+  const { data: sessions = [] } = useQuery<SessionSummary[]>({
+    queryKey: ["sessions"],
+    queryFn: listSessions,
+    refetchInterval: 10_000,
+  });
+
+  async function handleClick(id: string) {
+    await resumeSession(id);
+    onSessionSelect(id);
+  }
+
+  return (
+    <ul className="flex flex-col gap-1">
+      {sessions.map((s) => (
+        <li key={s.id}>
+          <button
+            data-testid={`session-item-${s.id}`}
+            data-active={s.id === activeSessionId ? "true" : "false"}
+            onClick={() => handleClick(s.id)}
+            className={[
+              "w-full text-left rounded px-2 py-1 text-xs hover:bg-accent truncate",
+              s.id === activeSessionId ? "bg-accent font-semibold" : "",
+            ].join(" ")}
+          >
+            <span className="block truncate">{s.question || s.id}</span>
+            <span className="block text-muted-foreground text-[10px]">{s.status}</span>
+          </button>
+        </li>
+      ))}
+    </ul>
+  );
+}