fix: address P0/P1/P2 audit findings — security, reliability, CI, docs

P0 - Critical:
- config.py: add _mask() credential-redaction helper, SETTINGS_SCHEMA_VERSION,
  plaintext-storage warning; corrupt settings.json backed up before defaults
- gost_util.py: pin SHA256 for gost_3.2.6_windows_amd64.zip; verify before
  extraction; _add_defender_exclusion now logs warning on failure
- firewall.py: add emergency_disengage() for atexit/signal use
- service.py: register fw_emergency_disengage via atexit + SIGTERM/SIGINT;
  stop() calls emergency_disengage if thread hangs past 15s timeout
- app.py: wrap main() in top-level except with CTk error dialog + log
- LICENSE: add MIT license file

P1 - Important:
- app.py: switch to RotatingFileHandler (5 MB / 3 backups)
- config.py: settings_version + migrate(); save_settings() writes .bak before
  overwrite; _is_safe_https_url() strips RFC-1918 sources/ip_check_url
- service.py: threading.Lock on _settings; _signal_handler; graceful stop
- requirements.txt: pin exact versions; add cryptography==48.0.0
- .gitignore: add settings.json, credential JSON files, screenshot noise
- tray.py, dns_leak.py, gost_util.py: replace bare except:pass with logging
- CHANGELOG.md: document all session changes

P2 - Nice to have:
- .github/workflows/test.yml: CI on Python 3.10/3.11/3.12 windows-latest
- run.py: --version / -V flag
- docs/OPERATOR_RUNBOOK.md: emergency disengage, proxy leak, GOST, settings

Tests: 47/47 passed (python -m unittest discover -s tests -v)
Co-authored-by: Cursor <cursoragent@cursor.com>

proxy_chain_manager/app.py

@@ -89,11 +89,14 @@ from .vpn_detect import detect_vpn
 from .windows_task import install_logon_task, task_exists, uninstall_logon_task
 
 LOG_PATH = app_data_dir() / "proxy_chain_manager.log"
+from logging.handlers import RotatingFileHandler as _RotatingFileHandler  # noqa: E402
 logging.basicConfig(
     level=logging.INFO,
     format="%(asctime)s %(levelname)s %(name)s: %(message)s",
     handlers=[
-        logging.FileHandler(LOG_PATH, encoding="utf-8"),
+        _RotatingFileHandler(
+            LOG_PATH, maxBytes=5 * 1024 * 1024, backupCount=3, encoding="utf-8"
+        ),
         logging.StreamHandler(sys.stdout),
     ],
 )
@@ -142,6 +145,25 @@ def _ts() -> str:
 
 
 def main() -> None:
+    try:
+        _main_inner()
+    except Exception as _exc:  # noqa: BLE001
+        _log = logging.getLogger(__name__)
+        _log.exception("Unhandled exception in main loop")
+        try:
+            import tkinter.messagebox as _mb
+            _mb.showerror(
+                "Proxy God — Fatal Error",
+                f"An unexpected error occurred and the application must close.\n\n"
+                f"{type(_exc).__name__}: {_exc}\n\n"
+                f"Details have been written to:\n{LOG_PATH}",
+            )
+        except Exception:
+            pass
+        raise
+
+
+def _main_inner() -> None:
     ctk.set_appearance_mode("dark")
     ctk.set_default_color_theme("dark-blue")