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/search.py

@@ -1,6 +1,6 @@
 import click
 from urllib.parse import quote_plus
-from browser_cli.client import send_command, BrowserNotConnected
+from browser_cli.commands import _handle
 from rich.console import Console
 
 console = Console()
@@ -71,14 +71,7 @@ def _build_command(engine_key: str, help_text: str) -> click.Command:
     def _cmd(query, bg, window, group):
         terms = " ".join(query)
         url = ENGINES[engine_key].format(query=quote_plus(terms))
-        try:
-            send_command("navigate.open", {"url": url, "background": bg, "window": window, "group": group})
-        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)
+        _handle("navigate.open", {"url": url, "background": bg, "window": window, "group": group})
         suffix = f" in group '{group}'" if group else (f" in window '{window}'" if window else "")
         display = _DISPLAY_NAMES.get(engine_key, engine_key.capitalize())
         console.print(f"[green]Searching[/green] [cyan]{display}[/cyan]: {terms}{suffix}")