refactor(dry-F2): centralize shared UI primitives (relative-time, toast, download-status)

- utils/date.js: add formatRelative(iso, {future,nullText}); migrate 6 sites
  (SourceRow, SubscriptionsTab, SourceHealthDot, SchedulerStatusBar +
  thin adapters in BackupRunsTable/SystemActivityTab for their '—' null text).
  PostCard (30d->absolute) and CredentialCard (mo/y buckets) intentionally
  keep bespoke formatters.
- utils/toast.js: toast(opts) wraps the globalThis.window?.__fcToast?.(...)
  incantation; migrate 63 call sites across 24 files.
- utils/downloadStatus.js: single source for the download-event status enum
  -> label/color/icon; collapse the 3 duplicate maps (DownloadStatChips,
  DownloadsFilterPopover, DownloadEventRow).

Net -33 lines. Platform metadata was already centralized in platformColor.js.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/subscriptions/SchedulerStatusBar.vue

@@ -24,6 +24,7 @@
 
 <script setup>
 import { computed } from 'vue'
+import { formatRelative } from '../../utils/date.js'
 
 const props = defineProps({
   // { last_tick_at, next_due_at, due_now, auto_sources } | null
@@ -45,16 +46,8 @@ const health = computed(() => {
   return tickAgeMs.value <= STALE_MS ? 'ok' : 'stale'
 })
 
-function rel(ms) {
-  const s = Math.max(0, Math.floor(ms / 1000))
-  if (s < 60) return `${s}s ago`
-  if (s < 3600) return `${Math.floor(s / 60)}m ago`
-  if (s < 86400) return `${Math.floor(s / 3600)}h ago`
-  return `${Math.floor(s / 86400)}d ago`
-}
-
 const lastRanLabel = computed(() =>
-  tickAgeMs.value == null ? 'never' : rel(tickAgeMs.value),
+  formatRelative(props.status?.last_tick_at, { nullText: 'never' }),
 )
 
 const nextLabel = computed(() => {