fix(downloads): GIF review round 2 — cap as sent, shown length, retry, Join reason

The panels judge a GIF's length in whole milliseconds between the ends as sent, the server's own rule, and a refused length reads rounded up. A shared picture says it failed only once its retry failed too. Join's reason shows on a key's focus; the editor's failure toasts name GIFs. The edit route's cap is test-held, and DESIGN.md records the GIF UI.

backend/tests/test_edit_gif.py

@@ -66,6 +66,26 @@ def test_a_gif_edit_it_cannot_make_is_refused_with_its_reason(
     assert e.value.reason == reason
 
 
+@needs_ffmpeg
+def test_the_edit_route_says_a_gif_too_long_with_its_cap(
+    db, make_user, as_user, tmp_path, monkeypatch
+):
+    monkeypatch.setattr(settings, "files_root", str(tmp_path / "root"))
+    u = make_user()
+    c = as_user(u)
+    source = _source(db, u, tmp_path)
+    r = c.post(
+        "/api/downloads/edit",
+        json={
+            "source_job_id": source.id,
+            "edit_spec": {"trim": {"start_s": 0, "end_s": 30.5}, "gif": {"side": 480}},
+        },
+    )
+    assert r.status_code == 400, r.text
+    # The cap goes along: the refusal's sentence says it ("at most 30 seconds").
+    assert r.json()["detail"] == {"code": "edit", "reason": "gif_too_long", "limit_s": 30}
+
+
 @needs_ffmpeg
 def test_a_gif_edit_of_exactly_thirty_seconds_is_allowed(db, make_user, tmp_path, monkeypatch):
     monkeypatch.setattr(settings, "files_root", str(tmp_path / "root"))