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/downloadStatus.js

@@ -0,0 +1,21 @@
+// Single source of truth for the download-event status enum -> display
+// metadata (label/color/icon). Mirrors the backend ENUM
+// (pending|running|ok|error|skipped). Used by the stat chips, the filter
+// dropdown, and the event rows so they never drift apart.
+
+export const DOWNLOAD_STATUSES = [
+  { value: 'pending', label: 'Queued',    color: 'grey',    icon: 'mdi-clock-outline' },
+  { value: 'running', label: 'Running',   color: 'info',    icon: 'mdi-progress-clock' },
+  { value: 'ok',      label: 'Completed', color: 'success', icon: 'mdi-check-circle' },
+  { value: 'error',   label: 'Failed',    color: 'error',   icon: 'mdi-alert-circle' },
+  { value: 'skipped', label: 'Skipped',   color: 'warning', icon: 'mdi-skip-next' },
+]
+
+const _BY_VALUE = Object.fromEntries(DOWNLOAD_STATUSES.map((s) => [s.value, s]))
+
+export function downloadStatusMeta(value) {
+  return _BY_VALUE[value] || { value, label: value, color: 'grey', icon: 'mdi-help-circle' }
+}
+export const downloadStatusLabel = (v) => downloadStatusMeta(v).label
+export const downloadStatusColor = (v) => downloadStatusMeta(v).color
+export const downloadStatusIcon = (v) => downloadStatusMeta(v).icon