feat(libs): create library by name only; root_path derived server-side as BOOKS_DIR/SafeName(name), validated at creation; drop root_path input from admin UI; align changelogs to bilingual template; trim AGENTS.md to rules not derivable from repo layout

frontend/src/api/client.ts

@@ -124,8 +124,7 @@ export const api = {
   deleteUser: (id: number) => apiFetch<void>(`/users/${id}`, { method: "DELETE" }),
 
   listLibraries: () => apiFetch<Library[]>("/libraries"),
-  createLibrary: (name: string, root_path: string) =>
-    apiFetch<Library>("/libraries", { method: "POST", body: { name, root_path } }),
+  createLibrary: (name: string) => apiFetch<Library>("/libraries", { method: "POST", body: { name } }),
   scanLibrary: (id: number) => apiFetch<{ accepted: boolean }>(`/libraries/${id}/scan`, { method: "POST" }),
   uploadBook: (id: number, file: File) => {
     const fd = new FormData();

frontend/src/pages/admin/Libraries.tsx

@@ -9,15 +9,13 @@ export default function AdminLibraries() {
   const qc = useQueryClient();
   const libsQ = useQuery({ queryKey: ["libraries"], queryFn: api.listLibraries });
   const [name, setName] = useState("");
-  const [root, setRoot] = useState("");
   const fileRefs = useRef<Record<number, HTMLInputElement | null>>({});
 
   const create = useMutation({
-    mutationFn: () => api.createLibrary(name.trim(), root.trim()),
+    mutationFn: () => api.createLibrary(name.trim()),
     onSuccess: (lib) => {
       toast("ok", `库「${lib.name}」已创建`);
       setName("");
-      setRoot("");
       void qc.invalidateQueries({ queryKey: ["libraries"] });
     },
     onError: (e) => toast("err", e instanceof Error ? e.message : "建库失败"),
@@ -54,14 +52,13 @@ export default function AdminLibraries() {
       <div className="flex-1 overflow-y-auto p-4">
         <h1 className="mb-4 text-lg font-semibold">库管理</h1>
         <form onSubmit={submit} className={`${card} mb-6 flex max-w-2xl flex-wrap items-center gap-3 p-4`}>
-          <input className={input} placeholder="库名(如 comics)" value={name} onChange={(e) => setName(e.target.value)} />
           <input
             className={input + " flex-1"}
-            placeholder="root_path(服务端绝对路径,须位于 BOOKS_DIR 下,如 /data/books/comics)"
-            value={root}
-            onChange={(e) => setRoot(e.target.value)}
+            placeholder="库名(如 comics,服务端目录自动落在 BOOKS_DIR/<库名> 下)"
+            value={name}
+            onChange={(e) => setName(e.target.value)}
           />
-          <button className={btnPrimary} disabled={create.isPending || !name.trim() || !root.startsWith("/")}>
+          <button className={btnPrimary} disabled={create.isPending || !name.trim()}>
             {create.isPending ? "创建中…" : "建库"}
           </button>
         </form>