feat(client): fall back to pure-Python SRP when srp is unimportable

Wrap `import srp` so a missing C-extension (e.g. Termux/aarch64) transparently
loads _srp_pure as srp. No call-site changes — the shim mirrors the srp API.

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

cmd_chat/client/client.py

@@ -7,7 +7,10 @@ from pathlib import Path
 from typing import Optional
 from uuid import uuid4
 
+try:
     import srp
+except ImportError:  # no aarch64 wheel / C-ext build under Termux — use the shim
+    from . import _srp_pure as srp
 import requests
 from cryptography.fernet import Fernet
 from cryptography.hazmat.primitives.kdf.hkdf import HKDF