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>

app/providers.tsx

@@ -2,7 +2,8 @@
 
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import { SessionProvider } from "next-auth/react";
-import { useState } from "react";
+import { Toaster } from "sonner";
+import { useEffect, useState } from "react";
 
 export function Providers({ children }: { children: React.ReactNode }) {
   const [queryClient] = useState(
@@ -16,9 +17,29 @@ export function Providers({ children }: { children: React.ReactNode }) {
       })
   );
 
+  const [theme, setTheme] = useState<"light" | "dark">("dark");
+  useEffect(() => {
+    const sync = () => {
+      setTheme(document.documentElement.classList.contains("dark") ? "dark" : "light");
+    };
+    sync();
+    const obs = new MutationObserver(sync);
+    obs.observe(document.documentElement, { attributes: true, attributeFilter: ["class"] });
+    return () => obs.disconnect();
+  }, []);
+
   return (
     <SessionProvider>
-      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+      <QueryClientProvider client={queryClient}>
+        {children}
+        <Toaster
+          theme={theme}
+          position="bottom-right"
+          richColors
+          closeButton
+          toastOptions={{ duration: 4000 }}
+        />
+      </QueryClientProvider>
     </SessionProvider>
   );
 }