HTTP transport: drop WS server, offline send queue + dead-stream watchdog

Gateway (docs/19):
- Remove ws_server.py; frame dispatch factored into dispatch.py
- http_server: media upload/pull, pairing over HTTP
- protocol: media frames mirrored; tests + ws_probe updated for HTTP

App:
- HttpGateway: postFrame/uploadMedia/pullMedia no longer throw on
  network failure (PostResult ok=false / Result.failure) — uncaught
  SocketTimeoutException on Dispatchers.Default crashed the app
- GatewayClient: dead-stream watchdog (health probe every 10s, 2
  failures -> redial in ~20s instead of the 45s SSE read timeout);
  state flips to Reconnecting when the stream dies, restored from the
  last hello.ack on long-poll success; poke() + backoff reset on app
  resume (MainActivity.onResume)
- Offline sends: composer enabled while disconnected; a send with no
  response (status 0) stays queued (Pending) and is auto-resent on the
  next (re)connect after a 2s outbox-replay grace; gateway 4xx
  rejections fail the bubble (tap to retry, no auto-loop)
- ChatStore: echo-replace and thread-relocate also match Failed
  bubbles (POST response lost in a network drop); loadHistory dedupes
  local failed bubbles the server already has; failMessage()
- MainActivity: poke() on resume so a backgrounded app reconnects
  promptly instead of waiting out the backoff

gateway-plugin/protocol.py

@@ -26,11 +26,8 @@ PROTOCOL_VERSION = 1
 # ---------------------------------------------------------------------------
 
 # Pairing / lifecycle
-TYPE_HELLO = "hello"
 TYPE_HELLO_ACK = "hello.ack"
 TYPE_ERROR = "error"
-TYPE_PING = "ping"
-TYPE_PONG = "pong"
 
 # Chat
 TYPE_MESSAGE = "message"
@@ -82,12 +79,8 @@ TYPE_SYNC_DONE = "sync.done"
 TYPE_HISTORY = "history"
 
 # Media (M4)
-TYPE_MEDIA_UPLOAD_START = "media.upload.start"
-TYPE_MEDIA_UPLOAD_END = "media.upload.end"
 TYPE_MEDIA_UPLOAD_ACK = "media.upload.ack"
 TYPE_MEDIA_OFFER = "media.offer"
-TYPE_MEDIA_PULL = "media.pull"
-TYPE_MEDIA_PULL_END = "media.pull.end"
 
 # Push / notifications (M5)
 TYPE_NOTIFICATION = "notification"
@@ -749,7 +742,8 @@ def media_offer(
     thread_id: str | None = None,
     message_id: str | None = None,
 ) -> Frame:
-    """Event: the agent produced media the app can fetch via ``media.pull``.
+    """Event: the agent produced media the app can fetch via
+    ``GET /v1/media/{media_id}`` (docs/19 §19.15).
 
     ``message_id`` (optional) associates the offer with the assistant message
     it belongs to (the app falls back to the lane's last assistant message).
@@ -766,14 +760,9 @@ def media_offer(
     return Frame(type=TYPE_MEDIA_OFFER, chat_id=chat_id, thread_id=thread_id, payload=payload)
 
 
-def media_pull_end(ok: bool, *, id: int | None = None) -> Frame:
-    """Terminal frame of a ``media.pull`` binary stream."""
-    return Frame(type=TYPE_MEDIA_PULL_END, id=id, payload={"ok": ok})
-
-
 def media_upload_ack(ok: bool, media_ref: str, *, id: int | None = None) -> Frame:
-    """Response to ``media.upload.end``: the ref is cached and may be used in
-    a ``message.send`` ``media_refs``. Failures use ``error`` frames instead."""
+    """Response to ``POST /v1/media``: the ref is cached and may be used in a
+    ``message.send`` ``media_refs``. Failures use ``error`` frames instead."""
     return Frame(
         type=TYPE_MEDIA_UPLOAD_ACK,
         id=id,
@@ -783,10 +772,3 @@ def media_upload_ack(ok: bool, media_ref: str, *, id: int | None = None) -> Fram
 
 def error(code: str, message: str, *, id: int | None = None) -> Frame:
     return Frame(type=TYPE_ERROR, id=id, payload={"code": code, "message": message})
-
-
-def pong(ts: int | None = None) -> Frame:
-    payload: dict[str, Any] = {}
-    if ts is not None:
-        payload["ts"] = ts
-    return Frame(type=TYPE_PONG, payload=payload)