chore: remove dead TierPicker + strike resolved gotchas

TierPicker.vue is a 5-tier preset picker from before the arbitrary-
amount contribution redesign. Zero imports across app/ and server/ —
purely dead code (99 lines).

Strike two LAUNCH_READINESS bullets that describe already-fixed
issues: the "stale tier comment" in useMemberPayment.js (no `tier`
references remain in that file), and the SeriesPassPurchase auto-
refresh gotcha (fetchPassInfo() already runs after the success path
at line 318).

docs/LAUNCH_READINESS.md

@@ -144,7 +144,6 @@ See `docs/TODO.md` for:
 
 - **Admin edit does not sync Helcim `recurringAmount`.** `/admin/members/[id]` PUT writes `contributionAmount` direct to Mongo by design. Admins must PATCH Helcim manually. Worth surfacing in admin UI or docs.
 - **Cadence switch rejected on active subscriptions.** `update-contribution.post.js:184-189` refuses cadence changes mid-subscription; no UI toggle exists on `/member/account`. Adding cadence switch would require a Helcim subscription replacement flow, not a plain update.
-- **`SeriesPassPurchase.vue` doesn't auto-refresh after purchase.** (Observed 2026-04-21 during Phase 4 series-pass functional tests.) Component's local `$fetch` to `/api/series/{id}/tickets/available` fires on mount + `userEmail` watch, but isn't re-invoked after a successful purchase — the "already registered" state only appears on next navigation. Parent page calls `refreshNuxtData()` but the component doesn't participate in it. Fix: call `fetchPassInfo()` after the success toast in `handleSubmit`, or lift the fetch to `useAsyncData` so it can be refreshed from outside.
 - **S2 test fixture `id`/`slug` inconsistency.** (Local dev only.) Seeded S2 series has `id: 'test-s2-drop-in-allowed'` but `slug: 'test-s2-drop-in-allowed-series'`. Doesn't affect prod — fix the seed script if anyone re-runs fixtures and is confused why `id`-based Mongo queries return empty.
 
 ### Events-surface visual audit — deferred items (2026-04-21)
@@ -159,8 +158,6 @@ Context: Phase 4 audit against `docs/specs/events-visual-audit-findings.md` fixe
 
 ### Contribution-amount redesign — cosmetic cleanup (naming only, not behavior)
 - Rename admin members column header "Tier" → "Contribution" (`app/pages/admin/members/index.vue:265`).
-- Delete dead `app/components/TierPicker.vue`.
-- Update stale tier comment in `app/composables/useMemberPayment.js:59`.
 - Update error log message referencing "tier" in `server/api/members/update-contribution.post.js:221`.
 - Rename `handleUpdateTier` handler in `app/pages/member/account.vue`.