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/common/ErrorDetailModal.vue

@@ -33,6 +33,7 @@
 </template>
 
 <script setup>
+import { toast } from '../../utils/toast.js'
 import { computed, ref, watch } from 'vue'
 
 import { copyText } from '../../utils/clipboard.js'
@@ -119,7 +120,7 @@ async function onCopy () {
     if (copiedTimer) clearTimeout(copiedTimer)
     copiedTimer = setTimeout(() => { copied.value = false }, 1500)
   } catch (e) {
-    window.__fcToast?.({ text: `Copy failed: ${e.message}`, type: 'error' })
+    toast({ text: `Copy failed: ${e.message}`, type: 'error' })
   }
 }
 </script>