feat: say when the firmware is flashed over USB

Builds with one app slot (C6, debug, and the C5 to come) have nowhere
for /updateFirmware to write. It answered "Error updating firmware: "
and rebooted.

- ConfigSettings::otaSlot, from esp_ota_get_next_update_partition().
- The header sends otaSlot, default 1, so only those builds send it.
- /updateFirmware refuses with 400 and a reason, and doesn't stop the
  radio or reboot.
- The web UI's Update Firmware says to flash the onboard image over USB,
  with a backup first. Firmware before the field leaves it undefined,
  which keeps the uploader.

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

test/js/model.test.js

@@ -37,6 +37,19 @@ test('a field the sync left out is its default', () => {
     for (const g of m.groupList()) assert.ok(Array.isArray(g.linkedShades));
 });
 
+test('otaSlot is false only when the device says so', () => {
+    // The recording is from firmware before the field.
+    assert.equal(synced().header.otaSlot, undefined);
+    const schema = recordedSchema();
+    schema.add({ F: [['os', 'otaSlot', 'b', 1]] });
+    const header = deviceSections(schema).find((d) => d.header).header;
+    const m = new ProtoModel(schema);
+    m.applySync({ header: header });
+    assert.equal(m.header.otaSlot, true);
+    m.applySync({ header: schema.expand(Object.assign({ os: 0 }, schema.pack(header))) });
+    assert.equal(m.header.otaSlot, false);
+});
+
 test('the lists come in their sort order', () => {
     const m = synced();
     const orders = m.shadeList().map((s) => s.sortOrder);