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/WorkflowBar.tsx

@@ -0,0 +1,40 @@
+// frontend/src/shell/WorkflowBar.tsx
+import { useSessionStore } from "../store/sessionStore";
+
+const PHASES = [
+  { id: "F1", name: "Disambiguazione" },
+  { id: "F2", name: "Schema linking" },
+  { id: "F3", name: "Esplorazione" },
+  { id: "F4", name: "Piano SQL" },
+  { id: "F5", name: "Generazione SQL" },
+  { id: "F6", name: "Validazione" },
+  { id: "F7", name: "Revisione" },
+  { id: "F8", name: "Risultati" },
+];
+
+export function WorkflowBar() {
+  const currentPhase = useSessionStore((s) => s.currentPhase);
+
+  return (
+    <nav className="flex gap-1 overflow-x-auto">
+      {PHASES.map((p) => {
+        const isActive = currentPhase === p.id;
+        return (
+          <span
+            key={p.id}
+            data-testid={`phase-${p.id}`}
+            data-active={isActive ? "true" : "false"}
+            className={[
+              "px-2 py-0.5 rounded text-xs whitespace-nowrap",
+              isActive
+                ? "bg-primary text-primary-foreground font-bold"
+                : "text-muted-foreground border",
+            ].join(" ")}
+          >
+            {p.id}: {p.name}
+          </span>
+        );
+      })}
+    </nav>
+  );
+}