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/import.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'
 
@@ -37,7 +38,7 @@ export const useImportStore = defineStore('import', () => {
       settings.value = await api.patch('/api/settings/import', { body: patch })
     } catch (e) {
       settingsError.value = e.message
-      window.__fcToast?.({ text: `Settings save failed: ${e.message}`, type: 'error' })
+      toast({ text: `Settings save failed: ${e.message}`, type: 'error' })
       throw e
     }
   }
@@ -68,7 +69,7 @@ export const useImportStore = defineStore('import', () => {
       const label = mode === 'deep'
         ? 'Deep scan triggered (re-applying sidecar metadata + filling NULL phash/artist on existing rows)'
         : mode === 'verify' ? 'Library verify triggered' : 'Quick scan triggered'
-      window.__fcToast?.({ text: label, type: 'success' })
+      toast({ text: label, type: 'success' })
       await refreshStatus()
       // Re-poll twice over ~5s and produce an HONEST follow-up toast.
       // Operator-flagged 2026-05-25: the prior "no new files" message was
@@ -91,22 +92,22 @@ export const useImportStore = defineStore('import', () => {
         const refreshedCount = sameBatch.filter(t => t.status === 'complete' && t.result_image_id).length
         const newImported = sameBatch.filter(t => t.status === 'complete' && t.result_image_id && !t.error).length
         if (mode === 'deep' && sameBatch.length > 0) {
-          window.__fcToast?.({
+          toast({
             text: `Deep scan finished — ${sameBatch.length} file(s) processed`,
             type: 'info',
           })
         } else if (mode === 'quick' && sameBatch.length > 0) {
-          window.__fcToast?.({
+          toast({
             text: `Quick scan finished — ${newImported} new file(s) queued`,
             type: 'info',
           })
         } else {
-          window.__fcToast?.({ text: 'Library is up to date', type: 'info' })
+          toast({ text: 'Library is up to date', type: 'info' })
         }
       }, 2000)
     } catch (e) {
       triggerError.value = e.message
-      window.__fcToast?.({ text: `Scan failed: ${e.message}`, type: 'error' })
+      toast({ text: `Scan failed: ${e.message}`, type: 'error' })
       throw e
     }
   }