fix(clip): a clip refusal speaks the user's language, and a choice in the clip form no longer sends it

The backend refuses a clip with a structured reason, but the client showed every one as the generic
"couldn't make the clip": each reason now has its own message in both languages, held to the
backend's reasons in both directions by the contract test. A segmented toggle's and the priority
stepper's buttons had no type, so inside a form one of them submitted it.

frontend/src/lib/api/core.test.ts

@@ -245,6 +245,22 @@ describe("req — a structured refusal reads in the user's language", () => {
     expect(err.detail).toBeUndefined();
   });
 
+  it.each([
+    ["too_long", "clip.errors.too_long"],
+    ["bad_crop", "clip.errors.bad_crop"],
+  ])("says why a clip was not cut (%s)", async (reason, localized) => {
+    // It read as the generic "couldn't carry out that action" — a 10-minute cap no retry gets past.
+    vi.spyOn(globalThis, "fetch").mockResolvedValue(
+      new Response(JSON.stringify({ detail: { code: "clip", reason } }), {
+        status: 422,
+        headers: { "content-type": "application/json" },
+      }),
+    );
+    await expect(
+      req("/api/downloads/clip", { method: "POST" }, { quiet: true }),
+    ).rejects.toMatchObject({ detail: localized });
+  });
+
   it.each(["quota", "reconnect", "unavailable"])(
     "says what to do about an action YouTube refused (%s)",
     async (reason) => {