feat: notifications modal, admin dedup, and in-app notification preferences

- Replace bell dropdown with full-screen NotificationsModal (mirrors SearchModal pattern)
- Notifications visible to all logged-in users (not just admin)
- Admin users excluded from new-chapter fan-out (dedup vs Scrape Complete notification)
- Users with notify_new_chapters=false opted out of new-chapter in-app notifications
- Toggle in profile page to enable/disable in-app new-chapter notifications
- PATCH /api/profile endpoint to save notification preferences
- User-facing /notifications page (admin redirects to /admin/notifications)

backend/internal/storage/store.go

@@ -1639,3 +1639,56 @@ func (s *Store) ListPushSubscriptionsByBook(ctx context.Context, slug string) ([
 	}
 	return subs, nil
 }
+
+// NotifyUsersWithBook creates an in-app notification for every logged-in user
+// who has slug in their library. Errors for individual users are logged but
+// do not abort the loop. Returns the number of notifications created.
+func (s *Store) NotifyUsersWithBook(ctx context.Context, slug, title, message, link string) int {
+	userIDs, err := s.ListUserIDsWithBook(ctx, slug)
+	if err != nil || len(userIDs) == 0 {
+		return 0
+	}
+	var n int
+	for _, uid := range userIDs {
+		if createErr := s.CreateNotification(ctx, uid, title, message, link); createErr == nil {
+			n++
+		}
+	}
+	return n
+}
+// who have slug in their user_library. Used to fan-out new-chapter notifications.
+// Admin users and users who have opted out of in-app new-chapter notifications
+// (notify_new_chapters=false on app_users) are excluded.
+func (s *Store) ListUserIDsWithBook(ctx context.Context, slug string) ([]string, error) {
+	// Collect user IDs to skip: admins + opted-out users.
+	skipIDs := make(map[string]bool)
+	excludedItems, err := s.pb.listAll(ctx, "app_users", `role="admin"||notify_new_chapters=false`, "")
+	if err == nil {
+		for _, raw := range excludedItems {
+			var rec struct {
+				ID string `json:"id"`
+			}
+			if json.Unmarshal(raw, &rec) == nil && rec.ID != "" {
+				skipIDs[rec.ID] = true
+			}
+		}
+	}
+
+	filter := fmt.Sprintf("slug=%q&&user_id!=''", slug)
+	items, err := s.pb.listAll(ctx, "user_library", filter, "")
+	if err != nil {
+		return nil, fmt.Errorf("ListUserIDsWithBook: %w", err)
+	}
+	seen := make(map[string]bool)
+	var ids []string
+	for _, raw := range items {
+		var rec struct {
+			UserID string `json:"user_id"`
+		}
+		if json.Unmarshal(raw, &rec) == nil && rec.UserID != "" && !seen[rec.UserID] && !skipIDs[rec.UserID] {
+			seen[rec.UserID] = true
+			ids = append(ids, rec.UserID)
+		}
+	}
+	return ids, nil
+}