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/utils/toast.js

@@ -0,0 +1,8 @@
+// Centralized toast trigger. App.vue registers the actual snackbar on
+// window.__fcToast; this wraps the optional-chaining lookup so call sites
+// don't repeat `globalThis.window?.__fcToast?.(...)` everywhere.
+//
+// opts: { text: string, type?: 'success' | 'error' | 'info' | 'warning' }
+export function toast(opts) {
+  globalThis.window?.__fcToast?.(opts)
+}