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/settings/SystemActivityTab.vue

@@ -154,6 +154,7 @@
 import { computed, onMounted, onUnmounted, ref } from 'vue'
 
 import { useSystemActivityStore } from '../../stores/systemActivity.js'
+import { formatRelative as fmtRelative } from '../../utils/date.js'
 import ErrorDetailModal from '../common/ErrorDetailModal.vue'
 import QueuesTable from './QueuesTable.vue'
 
@@ -277,14 +278,7 @@ function formatDuration(ms) {
   return `${(ms / 60_000).toFixed(1)} min`
 }
 function formatRelative(iso) {
-  if (!iso) return '—'
-  const then = new Date(iso).getTime()
-  const now = Date.now()
-  const diff = Math.max(0, (now - then) / 1000)
-  if (diff < 60) return `${Math.floor(diff)}s ago`
-  if (diff < 3600) return `${Math.floor(diff / 60)}m ago`
-  if (diff < 86400) return `${Math.floor(diff / 3600)}h ago`
-  return `${Math.floor(diff / 86400)}d ago`
+  return fmtRelative(iso, { nullText: '—' })
 }
 </script>