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/CentroidRecomputeCard.vue

@@ -16,6 +16,7 @@
 </template>
 
 <script setup>
+import { toast } from '../../utils/toast.js'
 import { ref } from 'vue'
 import { useMLStore } from '../../stores/ml.js'
 const store = useMLStore()
@@ -24,7 +25,7 @@ const done = ref(false)
 async function run() {
   busy.value = true
   try { await store.triggerRecomputeCentroids(); done.value = true }
-  catch (e) { window.__fcToast?.({ text: e.message, type: 'error' }) }
+  catch (e) { toast({ text: e.message, type: 'error' }) }
   finally { busy.value = false }
 }
 </script>