v1.2.0 + v1.3.0: Localization, About dialog, TOTP/2FA, stability improvements

v1.2.0:
- GUI localization (EN/RU/ZH) with language switcher and persistent selection
- About dialog (ⓘ) with app info, features, quick start guide
- core/i18n.py — internationalization module with t() function
- All GUI components translated via t() keys

v1.3.0:
- TOTP/2FA tab — Google Authenticator compatible codes with live 30s countdown,
  one-click copy, per-server secret management
- core/totp.py — TOTP module (pyotp, RFC 6238)
- core/logger.py — rotating file logger (5MB, 3 backups)
- Stronger Fernet encryption key with automatic migration from old key
- Thread-safe server store with locks, atomic writes, auto-restore on corruption
- Parallel status checks via ThreadPoolExecutor (up to 10 concurrent)
- SSH client: explicit channel cleanup, Unix key permissions
- Server dialog: port validation (1-65535), TOTP secret field
- Language change preserves active tab and server selection
- pyotp dependency added

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

core/logger.py

@@ -0,0 +1,36 @@
+"""
+Logging framework — rotating file log + console.
+"""
+
+import os
+import logging
+import logging.handlers
+
+SHARED_DIR = os.path.expanduser("~/.server-connections")
+LOG_FILE = os.path.join(SHARED_DIR, "app.log")
+
+
+def get_logger(name: str = "servermanager") -> logging.Logger:
+    """Get or create a named logger with file + console handlers."""
+    logger = logging.getLogger(name)
+    if logger.handlers:
+        return logger
+
+    logger.setLevel(logging.DEBUG)
+
+    # File handler — rotating, 5MB max, 3 backups
+    os.makedirs(SHARED_DIR, exist_ok=True)
+    fh = logging.handlers.RotatingFileHandler(
+        LOG_FILE, maxBytes=5 * 1024 * 1024, backupCount=3, encoding="utf-8"
+    )
+    fh.setLevel(logging.DEBUG)
+    fh.setFormatter(logging.Formatter(
+        "%(asctime)s [%(levelname)s] %(name)s: %(message)s",
+        datefmt="%Y-%m-%d %H:%M:%S"
+    ))
+    logger.addHandler(fh)
+
+    return logger
+
+
+log = get_logger()