refactor: reorganize client transport and extension internals

- Split client, native, remote, serve, markdown, and SDK internals into focused packages with direct imports.
- Move local and remote transport framing/protocol helpers behind clearer module boundaries.
- Break up the extension injected DOM logic into a separate content dispatch bundle and dedicated content modules.
- Add explicit client handling for passive remote discovery without noisy PQ warnings.
- Keep behavior covered with updated unit, integration, and extension tests.

browser_cli/endpoints.py

@@ -2,17 +2,15 @@
 
 Pure helpers (no sockets, no I/O) for turning user-facing ``host[:port]``
 strings into the canonical forms the rest of the client uses, and back into the
-short forms shown to humans. Re-exported from :mod:`browser_cli.client` for
-backward compatibility.
+short forms shown to humans.
 """
 from __future__ import annotations
 
 import re
 
+from browser_cli.constants import DEFAULT_REMOTE_PORT
 from browser_cli.errors import BrowserNotConnected
 
-_DEFAULT_REMOTE_PORT = 443
-
 def _looks_like_domain(host: str) -> bool:
     """True if host looks like a domain name rather than an IP address or localhost."""
     if host in {"localhost", "127.0.0.1", "::1"}:
@@ -35,7 +33,7 @@ def _resolve_connect_endpoint(endpoint: str) -> str:
     _, sep, _ = endpoint.rpartition(":")
     if not sep:
         if _looks_like_domain(endpoint):
-            return f"{endpoint}:{_DEFAULT_REMOTE_PORT}"
+            return f"{endpoint}:{DEFAULT_REMOTE_PORT}"
         raise BrowserNotConnected(
             f"Invalid remote endpoint '{endpoint}': expected host:port"
         )