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/SourceHealthDot.vue

@@ -22,6 +22,7 @@
 
 <script setup>
 import { computed } from 'vue'
+import { formatRelative } from '../../utils/date.js'
 
 const props = defineProps({
   source: { type: Object, required: true },
@@ -38,28 +39,13 @@ const level = computed(() => {
 
 const ariaLabel = computed(() => `source health: ${level.value}`)
 
-function relativeTime(iso) {
-  if (!iso) return 'Never'
-  const then = new Date(iso).getTime()
-  const now = Date.now()
-  const diff = Math.abs(now - then) / 1000
-  if (diff < 60) return `${Math.floor(diff)}s`
-  if (diff < 3600) return `${Math.floor(diff / 60)}m`
-  if (diff < 86400) return `${Math.floor(diff / 3600)}h`
-  return `${Math.floor(diff / 86400)}d`
-}
+const lastCheckedText = computed(() => formatRelative(props.source.last_checked_at))
 
-const lastCheckedText = computed(() => {
-  if (!props.source.last_checked_at) return 'Never'
-  return `${relativeTime(props.source.last_checked_at)} ago`
-})
-
-const nextCheckText = computed(() => {
-  if (!props.source.next_check_at) return null
-  const due = new Date(props.source.next_check_at).getTime()
-  if (due <= Date.now()) return 'imminent'
-  return `in ~${relativeTime(props.source.next_check_at)}`
-})
+const nextCheckText = computed(() =>
+  props.source.next_check_at
+    ? formatRelative(props.source.next_check_at, { future: true })
+    : null,
+)
 
 const truncatedError = computed(() => {
   const e = props.source.last_error || ''