fix: review fixes for the OTA slot

- The field is hasOtaSlot, like hasEthernet and the other "has" bools.
- The web UI shows the device's reason for any failed upload, not only
  for /restore, so it shows "no OTA slot" from a page that missed the
  header. The update routes' 500 replies give Update.errorString()
  instead of an empty reason.
- Native: the writer leaves hasOtaSlot out when true and writes "os":0.
- e2e: header() next to uptime(); the test skips on firmware without
  the field instead of failing, and matches the reason loosely. The
  README says what the test sends.
- PROTOCOL.md and the comments name only the builds that exist.

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

test/e2e/test_settings.py

@@ -86,9 +86,8 @@ def test_a_save_tells_the_other_clients(connect, schema):
 def test_a_wifi_scan_lists_the_networks(connect, schema):
     c = connect(schema=schema.hash)
     _, frames, _ = run_sync(c)
-    header = next(schema.expand(f["d"][schema.keys["header"]]) for f in frames if schema.keys["header"] in f["d"])
     # The connection in use: Ethernet with WiFi as the fallback may be on either.
-    if header.get("connType") != "Wifi":
+    if header(schema, frames).get("connType") != "Wifi":
         pytest.skip("the device isn't on WiFi")
     assert is_ok(schema, op(c, schema, "scanWifi", {}))
     found = c.recv_until(lambda f: f["t"] == schema.type_numbers["wifiNetworks"], timeout=20)[-1]
@@ -152,6 +151,12 @@ def wait_for_restart(host, token, schema, uptime_before):
         time.sleep(2)
 
 
+def header(schema, frames):
+    """The header a sync starts with."""
+    key = schema.keys["header"]
+    return next(schema.expand(f["d"][key]) for f in frames if key in f["d"])
+
+
 def uptime(schema, frames):
     """The uptime, in ms, from the status a sync ends with."""
     key = schema.keys["status"]