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/dns_leak.py

@@ -95,8 +95,8 @@ def get_system_dns_servers() -> list[str]:
         raw = (r.stdout or "").strip()
         if raw:
             return [x.strip() for x in raw.replace(";", ",").split(",") if x.strip()]
-    except Exception:
-        pass
+    except Exception as exc:
+        log.debug("PowerShell DNS server query failed (will try ipconfig): %s", exc)
 
     # ipconfig fallback
     try:
@@ -162,8 +162,8 @@ def _resolve_via_proxy(hostname: str, proxy_url: str, timeout: float = 8.0) -> l
                 data = r.json()
                 return [ans["data"] for ans in (data.get("Answer") or [])
                         if ans.get("type") == 1]
-    except Exception:
-        pass
+    except Exception as exc:
+        log.debug("DoH DNS leak query failed: %s", exc)
     return []