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/modal.js

@@ -1,4 +1,5 @@
 import { defineStore } from 'pinia'
+import { toast } from '../utils/toast.js'
 import { ref, computed } from 'vue'
 import { useApi } from '../composables/useApi.js'
 
@@ -96,7 +97,7 @@ export const useModalStore = defineStore('modal', () => {
       })
     } catch (e) {
       current.value.tags = prev
-      window.__fcToast?.({ text: `Failed to remove tag: ${e.message}`, type: 'error' })
+      toast({ text: `Failed to remove tag: ${e.message}`, type: 'error' })
       throw e
     }
   }