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/subscriptions/MaintenanceMenu.vue

@@ -31,6 +31,7 @@
 </template>
 
 <script setup>
+import { toast } from '../../utils/toast.js'
 import { useImportStore } from '../../stores/import.js'
 
 const emit = defineEmits(['refresh'])
@@ -39,10 +40,10 @@ const importStore = useImportStore()
 async function onRetry() {
   try {
     await importStore.retryFailed()
-    globalThis.window?.__fcToast?.({ text: 'Retry queued', type: 'success' })
+    toast({ text: 'Retry queued', type: 'success' })
     emit('refresh')
   } catch (e) {
-    globalThis.window?.__fcToast?.({ text: `Retry failed: ${e?.detail || e?.message || e}`, type: 'error' })
+    toast({ text: `Retry failed: ${e?.detail || e?.message || e}`, type: 'error' })
   }
 }
 
@@ -50,13 +51,13 @@ async function onClear() {
   try {
     const body = await importStore.clearStuck()
     const n = body?.cleared ?? 0
-    globalThis.window?.__fcToast?.({
+    toast({
       text: n ? `Cleared ${n} stuck task${n === 1 ? '' : 's'}` : 'Nothing stuck',
       type: 'success',
     })
     emit('refresh')
   } catch (e) {
-    globalThis.window?.__fcToast?.({ text: `Clear failed: ${e?.detail || e?.message || e}`, type: 'error' })
+    toast({ text: `Clear failed: ${e?.detail || e?.message || e}`, type: 'error' })
   }
 }
 </script>