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>

src/ProtoSchema.h

@@ -169,7 +169,7 @@
   FIELD(hostname, "hn", 's', 0) \
   FIELD(model, "md", 's', 0) \
   FIELD(chipModel, "cp", 's', 0) \
-  FIELD(otaSlot, "os", 'b', 1) \
+  FIELD(hasOtaSlot, "os", 'b', 1) \
   FIELD(connType, "ct", 's', 0) \
   /* memory, wifi and ethernet */ \
   FIELD(freeHeap, "fr", 'i', 0) \