Run browser file transcription through the long-form pipeline when asked and the VAD model is available

Adds an Advanced "Skip silence in uploaded files" option (off by default
pending measurement), the vad/longForm protocol fields, the old-path
fallback when the VAD model is missing, and a plain message for files
with no speech.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/engine.test.ts

@@ -77,6 +77,15 @@ describe("BrowserEngine", () => {
     expect("modelId" in message).toBe(false);
   });
 
+  test("forwards the long-form request only when set (D-WS3a)", () => {
+    const worker = new FakeWorker();
+    const engine = new BrowserEngine(worker);
+    void engine.transcribe(new Float32Array(4), { longForm: true });
+    void engine.transcribe(new Float32Array(4));
+    expect(worker.posted[0]!.message.longForm).toBe(true);
+    expect("longForm" in worker.posted[1]!.message).toBe(false);
+  });
+
   test("asks the worker to stream only when a consumer listens", () => {
     const worker = new FakeWorker();
     const engine = new BrowserEngine(worker);