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/SteerInput.tsx

@@ -0,0 +1,44 @@
+// frontend/src/shell/SteerInput.tsx
+import { useState } from "react";
+import { postSteer } from "../api/sessions";
+
+interface Props {
+  sessionId: string;
+}
+
+export function SteerInput({ sessionId }: Props) {
+  const [text, setText] = useState("");
+
+  async function submit() {
+    const trimmed = text.trim();
+    if (!trimmed) return;
+    await postSteer(sessionId, trimmed);
+    setText("");
+  }
+
+  function handleKeyDown(e: React.KeyboardEvent<HTMLInputElement>) {
+    if (e.key === "Enter") {
+      e.preventDefault();
+      submit();
+    }
+  }
+
+  return (
+    <div className="flex gap-2 items-center border-t pt-2">
+      <input
+        type="text"
+        value={text}
+        onChange={(e) => setText(e.target.value)}
+        onKeyDown={handleKeyDown}
+        placeholder="Steering libero (!)…"
+        className="flex-1 border rounded px-2 py-1 text-sm focus:outline-none focus:ring-1"
+      />
+      <button
+        onClick={submit}
+        className="border rounded px-3 py-1 text-sm hover:bg-accent"
+      >
+        Invia
+      </button>
+    </div>
+  );
+}