refactor: split compat into package, harden serve proxy (v0.9.3)

- compat.py → compat/ package: auth.py (auth-field normalizers),
  commands.py (command-format shims), __init__.py (re-exports)
- Add _auth_0_9_3 transformer: normalizes pubkey to lowercase before auth
  so clients < 0.9.3 sending uppercase hex are accepted
- adapt_auth() now called before auth check in serve.py; command extracted
  after adapt_auth so future transformers can rename commands safely
- serve.py: deduplicate _recv_exact (import from client), unify
  resp/resp_payload across Windows/Unix branches, require lowercase hex
  pubkey (re.fullmatch), reorganize imports, drop unused os import
- client.py: move payload/framed construction inside branches (remote path
  no longer serializes JSON it never uses); fix _is_valid_key_spec
  operator precedence; import MAX_MSG_BYTES from version_manager
- auth.py: narrow except clause (ValueError instead of bare Exception)
- Bump version 0.9.2 → 0.9.3

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

browser_cli/commands/windows.py

@@ -1,46 +1,11 @@
 import click
-from browser_cli.client import BrowserNotConnected, active_browser_targets, remote_browser_targets, send_command
+from browser_cli.commands import _handle, _handle_multi, _multi_browser_targets
 from rich.console import Console
 from rich.table import Table
 
 console = Console()
 
 
-def _handle(command, args=None, profile=None):
-    try:
-        return send_command(command, args or {}, profile=profile)
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
-def _handle_multi(command, args=None, profile=None, remote=None):
-    try:
-        if remote:
-            return send_command(command, args or {}, profile=profile, remote=remote)
-        return send_command(command, args or {}, profile=profile)
-    except (BrowserNotConnected, RuntimeError):
-        return None
-
-
-def _multi_browser_targets():
-    root = click.get_current_context().find_root()
-    if root.obj.get("browser_explicit"):
-        return []
-    remote = root.obj.get("remote")
-    key = root.obj.get("key")
-    if remote:
-        targets = remote_browser_targets(remote, key=key)
-    else:
-        targets = active_browser_targets(key=key)
-    if len(targets) <= 1 and not any(target.remote for target in targets):
-        return []
-    return targets
-
-
 def _print_windows(windows: list[dict], *, show_browser: bool = False) -> None:
     if not windows:
         console.print("[yellow]No windows found[/yellow]")