feat(soundtouch-web): add multi-room zone management

Backend:
- GET /api/zone/{id} — zone info enriched with device names and role flags
- POST /api/zone/{id}/add/{slaveId} — add slave (creates zone if standalone)
- POST /api/zone/{id}/remove/{slaveId} — remove slave from zone
- POST /api/zone/{id}/dissolve — dissolve zone to standalone
- POST /api/zone/{id}/leave — slave leaves its zone (backend finds master)

Frontend (Zone.js):
- Standalone: shows "Group with…" button, opens device picker overlay
- Master: member list with per-row Remove, Add speaker, Dissolve buttons
- Slave: shows master name, Leave zone button
- Lazy-loads on device detail open; refreshes after each zone operation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/soundtouch-web/spa_test.go

@@ -348,4 +348,4 @@ func TestJSONAPIConsistency(t *testing.T) {
 			}
 		})
 	}
-}
+}