fix(sftp): keep two rate samples when a chunk exceeds the window

ui/koko/tests/sftp/fileTransferRate.test.ts

@@ -22,7 +22,7 @@ describe("file transfer rate helper", () => {
     expect(remainingSeconds(10_240, 2_048, 2048)).toBe(4);
   });
 
-  it("drops stale samples and ignores zero-size remaining", () => {
+  it("keeps the last two samples when a chunk is slower than the window", () => {
     const samples = pushTransferRateSample(
       [
         { t: 1_000, bytes: 0 },
@@ -31,7 +31,11 @@ describe("file transfer rate helper", () => {
       200,
       7_000
     );
-    expect(samples).toEqual([{ t: 7_000, bytes: 200 }]);
+    expect(samples).toEqual([
+      { t: 2_000, bytes: 100 },
+      { t: 7_000, bytes: 200 }
+    ]);
+    expect(bytesPerSecond(samples)).toBe(20);
     expect(remainingSeconds(0, 0, 100)).toBeNull();
     expect(remainingSeconds(100, 100, 50)).toBe(0);
   });

ui/koko/utils/file-transfer/rate.ts

@@ -10,7 +10,10 @@ export function pushTransferRateSample(
   bytes: number,
   now = Date.now()
 ): TransferRateSample[] {
-  return [...samples, { t: now, bytes }].filter((sample) => now - sample.t <= sampleWindowMs).slice(-12);
+  const next = [...samples, { t: now, bytes }];
+  const recent = next.filter((sample) => now - sample.t <= sampleWindowMs);
+  if (recent.length >= 2) return recent.slice(-12);
+  return next.slice(-2);
 }
 
 export function bytesPerSecond(samples: TransferRateSample[] | null | undefined): number | null {