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/platform.py

@@ -2,9 +2,7 @@ import os
 import sys
 from pathlib import Path
 
-APP_NAME = "browser-cli"
-RUNTIME_DIRNAME = ".browser_cli"
-DEFAULT_ALIAS = "default"
+from browser_cli.constants import APP_NAME, DEFAULT_ALIAS, RUNTIME_DIRNAME
 
 def is_windows() -> bool:
     return sys.platform.startswith("win")