fix(downloads): a GIF's size warning judges the size it shows

frontend/src/lib/gif.test.ts

@@ -1,5 +1,11 @@
 import { describe, expect, it } from "vitest";
-import { estimateGif, GIF_MAX_BYTES, GIF_PREVIEW_MAX_BYTES, gifVerdict } from "./gif";
+import {
+  estimateGif,
+  gifEstimateText,
+  GIF_MAX_BYTES,
+  GIF_PREVIEW_MAX_BYTES,
+  gifVerdict,
+} from "./gif";
 
 const MB = 1024 * 1024;
 
@@ -34,9 +40,17 @@ describe("gifVerdict", () => {
   });
 
   it("warns past the server's cap, then past the link preview's", () => {
-    expect(gifVerdict(10, { hi: GIF_MAX_BYTES + 1 })).toBe("may_fail");
+    expect(gifVerdict(10, { hi: GIF_MAX_BYTES + 0.6 * MB })).toBe("may_fail");
     expect(gifVerdict(10, { hi: GIF_MAX_BYTES })).toBe("no_preview");
-    expect(gifVerdict(10, { hi: GIF_PREVIEW_MAX_BYTES + 1 })).toBe("no_preview");
+    expect(gifVerdict(10, { hi: GIF_PREVIEW_MAX_BYTES + 0.6 * MB })).toBe("no_preview");
     expect(gifVerdict(10, { hi: GIF_PREVIEW_MAX_BYTES })).toBe("ok");
   });
+
+  it("judges the size the reader is shown: a high end shown as 30 MB is not past 30 MB", () => {
+    // 30 s at 480 lands a hair over 30 MiB, and reads "about 12–30 MB".
+    const hi = GIF_MAX_BYTES + 0.4 * MB;
+    expect(gifEstimateText({ lo: 12 * MB, hi }, "en").hi).toBe("30");
+    expect(gifVerdict(30, { hi })).toBe("no_preview");
+    expect(gifVerdict(10, { hi: GIF_PREVIEW_MAX_BYTES + 0.4 * MB })).toBe("ok");
+  });
 });

frontend/src/lib/gif.ts

@@ -54,7 +54,10 @@ export type GifVerdict = "too_long" | "may_fail" | "no_preview" | "ok";
 
 export function gifVerdict(seconds: number, est: { hi: number }): GifVerdict {
   if (seconds > GIF_MAX_S + 0.0005) return "too_long";
-  if (est.hi > GIF_MAX_BYTES) return "may_fail";
+  // Judged on the high end as shown (whole megabytes): "about 12–30 MB" beside "it may pass 30 MB"
-  if (est.hi > GIF_PREVIEW_MAX_BYTES) return "no_preview";
+  // would contradict itself, and the estimate is no finer than that anyway.
+  const hi = Math.round(est.hi / MB) * MB;
+  if (hi > GIF_MAX_BYTES) return "may_fail";
+  if (hi > GIF_PREVIEW_MAX_BYTES) return "no_preview";
   return "ok";
 }