test(e2e): expand coverage and harden cross-file isolation

New specs (4):
- accept-invite: pre-registrant flow happy path + cadence/preset UX
- admin-pre-registrants: list, filter, action gating, redirect
- admin-series: list, create, edit (delete skipped — button no-ops)
- admin-site-content: list whitelist, edit + roundtrip on /

Extended specs (6):
- join-flow: cadence ×12 math, guidance label, paid-tier success
- events: series-pass-required, member-savings gating
- admin-events: full CRUD via /admin/events/create?edit=<id>
- admin-members: add-member submit, status select, detail nav
- a11y: add /accept-invite, /member/account, /board, /admin/pre-registrants
- wave-slack-onboarding: 9 of 16 scaffold tests now passing

Cross-file isolation hardening:
- admin-events CRUD: refresh auth cookie (auth.spec.js logout test
  bumps tokenVersion on the shared admin), wait for hydration
  before form fill, search by unique title to dodge pagination.
- board: switch memberPage from shared admin to dedicated seeded
  member to avoid the same tokenVersion race.
- wave-slack §6.4: create dedicated test member, filter by email
  before clicking, removing the "first row" anchor.

Also fixed board heading drift ("Board" → "Bulletin Board").

e2e/a11y.spec.js

@@ -7,16 +7,20 @@ const publicPages = [
   { name: "Join", path: "/join" },
   { name: "Events", path: "/events" },
   { name: "Coming Soon", path: "/coming-soon" },
+  { name: "Accept Invite", path: "/accept-invite" },
 ];
 
 const memberPages = [
   { name: "Member Dashboard", path: "/member/dashboard" },
   { name: "Member Profile", path: "/member/profile" },
+  { name: "Member Account", path: "/member/account" },
+  { name: "Board", path: "/board" },
 ];
 
 const adminPages = [
   { name: "Admin Members", path: "/admin/members" },
   { name: "Admin Events Create", path: "/admin/events/create" },
+  { name: "Admin Pre-Registrants", path: "/admin/pre-registrants" },
 ];
 
 test.describe("accessibility — public pages", () => {