refactor: one relative-time formatter, not three

MembershipRosterCard had grown its own ago(iso) helper, a near-copy of utils/date.js::formatRelative. C4's card was about to become a third copy before a hook caught it. Both now use the shared helper.

The hand-rolled copy was also slightly wrong in ways the shared one is not: it floored everything under a minute to '1m ago', and would have rendered NaNm ago for a null timestamp had a caller ever reached it without a v-if guard. Only sub-minute output changes, which no spec exercises - membershipRosterCard.spec.js seeds its rows at exactly 1h and 9d, where both helpers agree, and asserts on literal phrases rather than time strings.

Recorded formatRelative as snippet 3959 so the next component is offered it instead of deriving a fourth copy.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LNXXULQDjVZmbuNa2G9mD9

frontend/src/components/settings/MembershipRosterCard.vue

@@ -33,10 +33,10 @@
           never synced
         </span>
         <span v-else-if="!p.fresh" class="fc-roster__stale">
-          last synced {{ ago(p.last_success_at) }} — too old to rely on
+          last synced {{ formatRelative(p.last_success_at) }} — too old to rely on
         </span>
         <span v-else class="fc-roster__ok">
-          synced {{ ago(p.last_success_at) }}
+          synced {{ formatRelative(p.last_success_at) }}
         </span>
       </div>
 
@@ -47,7 +47,7 @@
           {{ p.last_count }} membership{{ p.last_count === 1 ? '' : 's' }} found
         </template>
         <template v-else-if="p.last_attempt_at">
-          tried {{ ago(p.last_attempt_at) }}, no successful sync yet
+          tried {{ formatRelative(p.last_attempt_at) }}, no successful sync yet
         </template>
       </div>
 
@@ -68,18 +68,12 @@
 <script setup>
 import { onMounted } from 'vue'
 
+import { formatRelative } from '../../utils/date.js'
 import { useMembershipSyncStore } from '../../stores/membershipSync.js'
 import MaintenanceTile from '../common/MaintenanceTile.vue'
 
 const store = useMembershipSyncStore()
 
-function ago (iso) {
-  const diff = (Date.now() - new Date(iso).getTime()) / 1000
-  if (diff < 3600) return `${Math.max(1, Math.floor(diff / 60))}m ago`
-  if (diff < 86400) return `${Math.floor(diff / 3600)}h ago`
-  return `${Math.floor(diff / 86400)}d ago`
-}
-
 async function refresh () {
   await store.syncNow()
   await store.load()