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/version_manager.py

@@ -1,7 +1,6 @@
 from importlib.metadata import version as _pkg_version
 
-PROTOCOL_MIN_CLIENT = "0.9.0"
-MAX_MSG_BYTES = 32 * 1024 * 1024
+from browser_cli.constants import MAX_MSG_BYTES, PROTOCOL_MIN_CLIENT
 
 def parse_version(v: str) -> tuple[int, ...]:
     try: