feat(member): collect member-overview bulk actions into a single dropdown

The growing row of bulk-action buttons above the member overview is replaced
by one "Aktionen" dropdown holding all four actions (open in email program,
copy addresses, export CSV, export PDF). With no selection the actions operate
on all — or the currently filtered — members; the email-program action is
disabled past a recipient cap, because the browser cannot reliably hand a very
long mailto over to the mail client. The trigger shows the active scope as a
badge: an emphasized count when members are selected, a muted "alle"/"gefiltert"
otherwise.

lib/mv/constants.ex

@@ -40,6 +40,8 @@ defmodule Mv.Constants do
 
   @max_boolean_filters 50
 
+  @max_mailto_bulk_recipients 50
+
   @max_uuid_length 36
 
   @email_validator_checks [:html_input, :pow]
@@ -173,6 +175,21 @@ defmodule Mv.Constants do
   """
   def max_boolean_filters, do: @max_boolean_filters
 
+  @doc """
+  Returns the maximum number of mailto recipients before the bulk "open in email
+  program" action is disabled.
+
+  The mailto link carries every recipient in its BCC; browsers cannot reliably
+  hand a too-long mailto URI to the mail program. At or above this count the
+  action is disabled in the UI (Copy and Export have no such limit).
+
+  ## Examples
+
+      iex> Mv.Constants.max_mailto_bulk_recipients()
+      50
+  """
+  def max_mailto_bulk_recipients, do: @max_mailto_bulk_recipients
+
   @doc """
   Returns the maximum length of a UUID string (36 characters including hyphens).