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/store/sessionStore.ts

@@ -11,9 +11,11 @@ interface SessionState {
   transcript: Entry[];
   toasts: { level: string; text: string }[];
   lastSystemEvent: StreamEvent | null;
+  currentPhase: string | null;
   applyEvent: (e: StreamEvent) => void;
   clearPending: () => void;
   resetSession: () => void;
+  setPhase: (phase: string | null) => void;
 }
 
 const empty = {
@@ -21,6 +23,7 @@ const empty = {
   transcript: [] as Entry[],
   toasts: [] as { level: string; text: string }[],
   lastSystemEvent: null,
+  currentPhase: null as string | null,
 };
 
 export const useSessionStore = create<SessionState>((set) => ({
@@ -41,4 +44,5 @@ export const useSessionStore = create<SessionState>((set) => ({
     }),
   clearPending: () => set({ pendingWidget: null }),
   resetSession: () => set({ ...empty }),
+  setPhase: (phase) => set({ currentPhase: phase }),
 }));