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/commands/__init__.py

@@ -13,11 +13,10 @@ from rich.table import Table
 
 from browser_cli import BrowserCLI, BrowserCounts
 from browser_cli.client import BrowserNotConnected
+from browser_cli.constants import GENTLE_MODES
 
 _console = Console()
 
-GENTLE_MODES = ["auto", "normal", "gentle", "ultra"]
-
 # Reusable ``--tab`` option: select a tab by ID (default: the active tab).
 tab_option = click.option("--tab", "tab_id", type=int, default=None, help="Tab ID (default: active tab)")