refactor(admin): split the backups section

The section becomes a composer over backups/: the restore dialog with its
preview, the refusal, skip and schedule notices, the restore jobs and
their request, the upload flow, the run and download actions, and the
two row layouts sharing one action pair. The upload total is a pure
function and tested as one. The read-failure guard's 250-character
window had stopped reaching the restore preview's own failure lines; the
preview now sits in a smaller component, the guard sees it, and it is
listed as a deliberate exception with its reason.

src/__tests__/baselines/source-function-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
-  "count": 401,
-  "total": 137456,
+  "count": 399,
+  "total": 136612,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 314,
@@ -80,8 +80,6 @@
     "components/admin/admin-shell.tsx :: AdminShell": 254,
     "components/admin/api-token-overview-section.tsx :: ApiTokenOverviewSection": 212,
     "components/admin/app-log-preview-section.tsx :: AppLogPreviewSection": 341,
-    "components/admin/backups-section.tsx :: BackupsSection": 618,
-    "components/admin/backups-section.tsx :: RestoreRowDialog": 226,
     "components/admin/central-codex-section.tsx :: CentralCodexSection": 241,
     "components/admin/encryption-section.tsx :: EncryptionSection": 303,
     "components/admin/general-settings-section.tsx :: GeneralSettingsSection": 220,