fix(launch-flow): align Slack-wait copy across join, dashboard, welcome email

- /join "How membership works" lists community (not Slack) as a benefit;
  adds a note that Slack invitations come in monthly onboarding waves.
- Dashboard slack-coming note drops "2–3 weeks" timeline; uses the same
  monthly-waves phrasing.
- Welcome email no longer points new members to Slack (which they don't
  yet have access to); directs them to reply instead.

app/pages/member/dashboard.vue

@@ -39,8 +39,8 @@
               <span>${{ memberData?.contributionAmount ?? 0 }} CAD/mo</span>
             </div>
             <p v-if="showSlackComingNote" class="slack-coming-note">
-              Slack workspace access is part of your membership. Your invitation
-              typically arrives within 2&ndash;3 weeks of joining.
+              Slack workspace access is part of your membership. Invitations are
+              sent in monthly onboarding waves &mdash; we'll be in touch.
             </p>
           </PageHeader>