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/credentials/ExtensionKeyBar.vue

@@ -26,6 +26,7 @@
 </template>
 
 <script setup>
+import { toast } from '../../utils/toast.js'
 import { onMounted, ref } from 'vue'
 import { useCredentialsStore } from '../../stores/credentials.js'
 import { copyText } from '../../utils/clipboard.js'
@@ -39,9 +40,9 @@ async function copyKey() {
   if (!store.extensionKey) return
   try {
     await copyText(store.extensionKey)
-    globalThis.window?.__fcToast?.({ text: 'Copied', type: 'success' })
+    toast({ text: 'Copied', type: 'success' })
   } catch {
-    globalThis.window?.__fcToast?.({ text: 'Copy failed', type: 'error' })
+    toast({ text: 'Copy failed', type: 'error' })
   }
 }
 
@@ -50,7 +51,7 @@ function confirmRotate() { showRotateConfirm.value = true }
 async function doRotate() {
   showRotateConfirm.value = false
   await store.rotateKey()
-  globalThis.window?.__fcToast?.({ text: 'Key rotated', type: 'success' })
+  toast({ text: 'Key rotated', type: 'success' })
 }
 </script>