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/stores/seriesManage.js

@@ -1,4 +1,5 @@
 import { defineStore } from 'pinia'
+import { toast } from '../utils/toast.js'
 import { ref } from 'vue'
 import { useApi } from '../composables/useApi.js'
 
@@ -59,7 +60,7 @@ export const useSeriesManageStore = defineStore('seriesManage', () => {
     })
     pickerSelection.value = []
     await refresh()
-    globalThis.window?.__fcToast?.({ text: 'Added to series', type: 'success' })
+    toast({ text: 'Added to series', type: 'success' })
   }
 
   async function remove(imageId) {