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>

src/index.css

@@ -519,6 +519,32 @@ body {
   flex-wrap: wrap;
 }
 
+/* --- AdvancedOptions (long-form "Skip silence", D-WS3a) --- */
+.advanced-options summary {
+  cursor: pointer;
+  color: var(--muted);
+  font-size: 0.85rem;
+}
+
+.advanced-options label {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  margin-top: 0.5rem;
+  font-size: 0.9rem;
+}
+
+.advanced-options input[type="checkbox"] {
+  accent-color: var(--accent);
+}
+
+.advanced-hint {
+  margin: 0.25rem 0 0;
+  color: var(--muted);
+  font-size: 0.85rem;
+  max-width: 60ch;
+}
+
 /* --- EngineToggle (M5, D-010) --- */
 
 .engine-toggle {