feat(ui): lock zone type and start audio from BPM

Zone preset vs sequence is fixed at create; edit shows read-only type.
Header BPM button starts beat detection when the detector is stopped.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/models/zone.py

@@ -134,7 +134,11 @@ class Zone(Model):
         id_str = str(id)
         if id_str not in self:
             return False
-        patch = data if isinstance(data, dict) else {}
+        patch = dict(data) if isinstance(data, dict) else {}
+        doc = self[id_str]
+        locked_kind = self._normalized_content_kind(doc) or self._infer_content_kind(doc)
+        if "content_kind" in patch:
+            patch["content_kind"] = locked_kind
         self[id_str].update(patch)
         if "content_kind" in patch:
             self._enforce_content_kind_invariants(self[id_str])