fix: language default, google OAuth cookie issue, add admin tier/usage controls

- ai-generate-dialog: useLocale() returns {locale,setLocale} object, not
  string — was stringifying whole object as default language value
- auth/server: add trustedOrigins so session cookie survives reverse-proxy
  deployments where BETTER_AUTH_URL differs from container's own view
- admin: add tier limit editor (/admin/tiers) and per-user usage reset
  button, both audit-logged

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/components/admin/reset-usage-button.tsx

@@ -0,0 +1,38 @@
+"use client";
+
+import { useState } from "react";
+import { useRouter } from "next/navigation";
+import { toast } from "sonner";
+import { Button } from "@/components/ui/button";
+
+export function ResetUsageButton({ userId }: { userId: string }) {
+  const router = useRouter();
+  const [resetting, setResetting] = useState(false);
+
+  async function handleReset() {
+    if (!confirm("Reset this user's usage counters for the current month?")) return;
+    setResetting(true);
+    try {
+      const res = await fetch(`/api/v1/admin/users/${userId}/usage`, { method: "PATCH" });
+      if (!res.ok) throw new Error("Reset failed");
+      toast.success("Usage reset");
+      router.refresh();
+    } catch {
+      toast.error("Failed to reset usage");
+    } finally {
+      setResetting(false);
+    }
+  }
+
+  return (
+    <Button
+      variant="outline"
+      size="sm"
+      onClick={() => { void handleReset(); }}
+      disabled={resetting}
+      className="text-destructive hover:text-destructive"
+    >
+      {resetting ? "Resetting…" : "Reset Usage"}
+    </Button>
+  );
+}