feat(server): persist story quest difficulty and configurable starting chapter

go/internal/server/deck/story_party.go

@@ -0,0 +1,26 @@
+package deck
+
+func (s *Store) CurrentFieldDeck() []FieldEntry {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return append([]FieldEntry(nil), s.state.FieldDeck...)
+}
+
+// SetStoryParty stores the authored active quest formation using the same
+// character ownership checks as DeckSave. Empty formation keeps player choice.
+func (s *Store) SetStoryParty(entries []DeckEntry) error {
+	if len(entries) == 0 {
+		return nil
+	}
+	if err := validDeck(entries); err != nil {
+		return err
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if err := s.validateOwnedDeckLocked(entries); err != nil {
+		return err
+	}
+	next := clone(s.state)
+	next.Deck = append([]DeckEntry(nil), entries...)
+	return s.commit(next)
+}