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/i18n/backendIds.contract.test.ts

@@ -170,6 +170,12 @@ const youtubeReasons = [
     backend("youtube/client.py").match(/^YouTubeRefusal = Literal\[([^\]]*)\]/m)?.[1] ?? ""
   ).matchAll(/"(\w+)"/g),
 ].map((m) => m[1]!);
+// A clip the server will not cut → `clip.errors.<reason>`: every `ClipError("<reason>")` in the backend.
+const clipReasons = [
+  ...new Set(
+    appSources().flatMap((src) => [...src.matchAll(/ClipError\("(\w+)"\)/g)].map((m) => m[1]!)),
+  ),
+];
 
 // The closed English vocabularies the backend WRITES into data, named through `vocab.<group>`
 // (lib/vocab) by their letters-and-digits key. A word the backend adds shows in English until named —
@@ -233,6 +239,7 @@ describe("backend ids have their names in both languages", () => {
     expect(goneWhats).toEqual(expect.arrayContaining(["playlist", "download", "job"]));
     expect(opaqueWhats).toEqual([]);
     expect(youtubeReasons).toEqual(["quota", "reconnect", "unavailable", "gone"]);
+    expect(clipReasons).toEqual(expect.arrayContaining(["too_long", "past_end"]));
     expect(crewJobs).toEqual(
       expect.arrayContaining(["Director", "Creator", "Director of Photography"]),
     );
@@ -336,6 +343,13 @@ describe("backend ids have their names in both languages", () => {
       ).toEqual([]);
     });
 
+    it(`${lang}: every reason a clip is refused, and none it cannot be`, () => {
+      expect(missing(clipReasons.map((r) => `clip.errors.${r}`))).toEqual([]);
+      expect([...idsUnder(keys, "clip.errors")].filter((r) => !clipReasons.includes(r))).toEqual(
+        [],
+      );
+    });
+
     it(`${lang}: every crew job, auto-tag and built-in profile the backend writes, and no other`, () => {
       for (const [group, words] of Object.entries(VOCAB)) {
         const wanted = words.map((w) => `vocab.${group}.${vocabKey(w)}`);