feat(downloads): a clip or an edit can be a GIF, shared as a picture

A clip and an editor range can come out as a silent GIF (long side
320/480/640, 12 fps, at most 30 s, one palette per GIF); one over 30 MB
is not kept. A shared picture is served inline under the preview's
sandbox, and a GIF's link card is the GIF itself up to 15 MB, the still
for Facebook's crawler.

frontend/src/lib/api/refusalText.dom.test.ts

@@ -55,6 +55,15 @@ describe.each(["en", "hu"])("localizeDetail (%s)", (lng) => {
     expect(upload).not.toBe(i18n.t("downloads.errors.upload.tooLarge", { size: "" }));
   });
 
+  it("says a clip's cap in minutes and a GIF's in seconds, from a clip or an edit", async () => {
+    expect(await say({ code: "clip", reason: "too_long", limit_s: 600 })).toContain("10");
+    for (const code of ["clip", "edit"]) {
+      const said = await say({ code, reason: "gif_too_long", limit_s: 30 });
+      expect(said).toContain("30");
+      expect(said).not.toContain("{{");
+    }
+  });
+
   it("says a shared-sentence code's own fallback for a reason it does not know", async () => {
     expect(await say({ code: "quota", reason: "nope" })).toBe(
       i18n.t("downloads.errors.quota.generic"),