feat: timezone="auto" derives the zone from the proxy egress IP

A proxy in a different country paired with the host timezone is the
classic timezone_mismatch signal, so a session with a proxy and no
explicit timezone now resolves the zone automatically.

- discover the egress IP through the proxy (SOCKS via requests[socks]),
  map it to an IANA zone with an offline mmdb (daijro/geoip-all-in-one,
  downloaded + cached like the Firefox binary; GPL so not vendored)
- precedence: explicit zone wins; ""+proxy and "auto"+proxy resolve;
  ""/"auto" without a proxy stay host; "host"/"local" force host TZ
- fail-early when a proxy is set but the zone cannot be resolved, never
  a silent host-TZ fallback
- deps: requests[socks], maxminddb, tzdata (zoneinfo ships no DB on Windows)
- resolve_session_timezone / ensure_geoip_mmdb exported for integrations

pyproject.toml

@@ -22,7 +22,9 @@ classifiers = [
 dependencies = [
     "playwright>=1.40",
     "platformdirs>=4",
-    "requests>=2.31",
+    "requests[socks]>=2.31",
+    "maxminddb>=2.2",
+    "tzdata>=2024.1",
     "tqdm>=4.66",
     "pywin32>=306; sys_platform == 'win32'",
 ]