Add AssetItem CSV import/export

/assets gets Export CSV (GET /api/assets/export) and Import CSV
(POST /api/assets/import) buttons. Export emits human-readable
manufacturer/model columns instead of raw modelId; import resolves those
back to an AssetModel (case-insensitive match, must already exist -- this
does not auto-create models) so export -> edit in Excel -> re-import is a
real round trip.

Per-row import errors (unmatched model, missing required field, bad date)
are collected and returned alongside the created count rather than
aborting the whole file on the first bad row -- a typo in row 200 should
not cost rows 1-199. warrantyEndDate still auto-fills from the models
warrantyMonths when the CSV omits it, same as manual creation.

Extracted src/lib/csv.ts (splitCsvLine/splitCsvLines/normalizeHeader/
csvField) out of expected-assets.ts so both CSV parsers share one
implementation instead of duplicating it.

Manual smoke test only (tsx script: export shape, valid row + unmatched-
model row + missing-date row all handled correctly), no Vitest coverage,
not through the ba->pm->feature-implementer->test-runner->code-reviewer
pipeline -- same deviation as the rest of this branch, see CLAUDE.md.
tsc/lint clean.

src/app/assets/page.tsx

@@ -77,6 +77,50 @@ export default function AssetsPage() {
   const [submitting, setSubmitting] = useState(false);
   const [submitError, setSubmitError] = useState<string | null>(null);
 
+  const [importStatus, setImportStatus] = useState<"idle" | "uploading" | "done" | "error">(
+    "idle"
+  );
+  const [importMessage, setImportMessage] = useState<string | null>(null);
+
+  async function handleImportChange(event: React.ChangeEvent<HTMLInputElement>) {
+    const file = event.target.files?.[0];
+    event.target.value = ""; // allow re-selecting the same file name after an error
+    if (!file) return;
+
+    setImportStatus("uploading");
+    setImportMessage(null);
+    try {
+      const text = await file.text();
+      const res = await fetch("/api/assets/import", {
+        method: "POST",
+        headers: { "Content-Type": "text/csv" },
+        body: text,
+      });
+      const body = await res.json();
+      if (!res.ok) {
+        setImportStatus("error");
+        setImportMessage(body.error ?? "Import thất bại");
+        return;
+      }
+      const errorCount = (body.errors as unknown[] | undefined)?.length ?? 0;
+      setImportStatus(errorCount > 0 ? "error" : "done");
+      const errorSummary =
+        errorCount > 0
+          ? " Lỗi: " +
+            (body.errors as { row: number; serialNumber: string; error: string }[])
+              .slice(0, 5)
+              .map((e) => `dòng ${e.row} (${e.serialNumber || "?"}): ${e.error}`)
+              .join("; ") +
+            (errorCount > 5 ? ` (+${errorCount - 5} lỗi khác)` : "")
+          : "";
+      setImportMessage(`Đã import ${body.created} asset.` + errorSummary);
+      refetch();
+    } catch {
+      setImportStatus("error");
+      setImportMessage("Không đọc được file -- kiểm tra lại định dạng CSV.");
+    }
+  }
+
   async function handleSubmit(event: FormEvent<HTMLFormElement>) {
     event.preventDefault();
     setSubmitting(true);
@@ -174,6 +218,49 @@ export default function AssetsPage() {
         )}
       </section>
 
+      <section className="flex max-w-2xl flex-col gap-3">
+        <h2 className="text-lg font-semibold">Import / Export CSV</h2>
+        <p className="text-sm text-muted-foreground">
+          Cột nhận diện theo tên (không phân biệt hoa/thường): <code>serial number</code>,{" "}
+          <code>manufacturer</code>, <code>model</code> (phải khớp Asset Model đã có sẵn ở{" "}
+          <a href="/asset-models" className="underline">
+            /asset-models
+          </a>
+          ), <code>purchase date</code> (bắt buộc), <code>warranty end date</code>,{" "}
+          <code>purchase order id</code> (tuỳ chọn -- bỏ trống thì tự tính từ warranty của model).
+        </p>
+        <div className="flex flex-wrap items-center gap-3">
+          <a href="/api/assets/export">
+            <Button type="button" variant="outline">
+              Export CSV
+            </Button>
+          </a>
+
+          <input
+            type="file"
+            accept=".csv,text/csv"
+            onChange={handleImportChange}
+            disabled={importStatus === "uploading"}
+            className="hidden"
+            id="assets-csv-input"
+          />
+          <Button asChild>
+            <label htmlFor="assets-csv-input" className="cursor-pointer">
+              {importStatus === "uploading" ? "Đang import..." : "Import CSV"}
+            </label>
+          </Button>
+        </div>
+        {importMessage ? (
+          <p
+            className={
+              importStatus === "error" ? "text-sm text-destructive" : "text-sm text-foreground"
+            }
+          >
+            {importMessage}
+          </p>
+        ) : null}
+      </section>
+
       <section className="flex max-w-sm flex-col gap-4">
         <h2 className="text-lg font-semibold">New Asset</h2>