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/settings/BrowserExtensionCard.vue

@@ -108,6 +108,7 @@
 </template>
 
 <script setup>
+import { toast } from '../../utils/toast.js'
 import { computed, onMounted, ref } from 'vue'
 import { useApi } from '../../composables/useApi.js'
 import { copyText } from '../../utils/clipboard.js'
@@ -147,7 +148,7 @@ async function loadKey() {
     apiKey.value = key
   } catch (e) {
     apiKey.value = ''
-    window.__fcToast?.({
+    toast({
       text: `Failed to load extension API key: ${e.message}`,
       type: 'error',
     })
@@ -160,9 +161,9 @@ async function rotateKey() {
     const { key } = await api.post('/api/settings/extension_api_key/rotate')
     apiKey.value = key
     keyShown.value = true
-    window.__fcToast?.({ text: 'Extension API key rotated.', type: 'success' })
+    toast({ text: 'Extension API key rotated.', type: 'success' })
   } catch (e) {
-    window.__fcToast?.({
+    toast({
       text: `Rotate failed: ${e.message}`,
       type: 'error',
     })
@@ -174,9 +175,9 @@ async function rotateKey() {
 async function copy(text, label) {
   try {
     await copyText(text)
-    window.__fcToast?.({ text: `${label} copied.`, type: 'success' })
+    toast({ text: `${label} copied.`, type: 'success' })
   } catch (e) {
-    window.__fcToast?.({ text: `Copy failed: ${e.message}`, type: 'error' })
+    toast({ text: `Copy failed: ${e.message}`, type: 'error' })
   }
 }
 </script>