UX polish pass 2: toasts, optimistic updates, mod update detection

- Install sonner; <Toaster> mounted in Providers, auto-tracks theme.
- Toasts replace inline result Alerts across ServerControls, PlayerManager,
  BackupManager, ModManager (install/remove/restore/delete/start/stop).
- PlayerManager: optimistic op/deop/whitelist/ban/pardon via onMutate +
  rollback; UI updates instantly before RCON round-trip.
- Modrinth search results now show author + "updated Xd ago" with full
  timestamp on hover; downloads on its own row.
- New /api/mods/updates endpoint: per-installed-mod Modrinth latest-version
  lookup (parallel, 30min memo). Amber "Update available" badge rendered
  next to installed mod rows when filenames differ.
- PlayerAvatar + Modrinth icons migrated to next/image (unoptimized, size
  hints) — fewer layout shifts.
- Login page surfaces ?error= + NextAuth error codes (CredentialsSignin,
  SessionRequired, etc.), preserves callbackUrl, adds autocomplete hints
  and role="alert". Wrapped in Suspense per Next 16 requirement.
- Snapshots + backups show relative "Xh ago" with exact timestamp on hover
  via new lib/time.ts helper.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -23,6 +23,7 @@
     "react": "19.2.4",
     "react-dom": "19.2.4",
     "shadcn": "^4.2.0",
+    "sonner": "^2.0.7",
     "tailwind-merge": "^3.5.0",
     "tw-animate-css": "^1.4.0"
   },