fix: improve mailto BCC encoding

Use URI.encode_www_form() instead of URI.encode() for mailto query parameters.
This is the safer choice for query parameter encoding.

Add comment about mailto URL length limits that vary by email client.

lib/mv_web/live/member_live/index.ex

@@ -1126,6 +1126,9 @@ defmodule MvWeb.MemberLive.Index do
 
   # Updates selection-related assigns (selected_count, any_selected?, mailto_bcc)
   # to avoid recalculating Enum.any? and Enum.count multiple times in templates.
+  #
+  # Note: Mailto URLs have length limits that vary by email client.
+  # For large selections, consider using export functionality instead.
   defp update_selection_assigns(socket) do
     members = socket.assigns.members
     selected_members = socket.assigns.selected_members
@@ -1140,6 +1143,7 @@ defmodule MvWeb.MemberLive.Index do
       if any_selected? do
         format_selected_member_emails(members, selected_members)
         |> Enum.join(", ")
+        |> URI.encode_www_form()
       else
         ""
       end