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/totp.py

@@ -0,0 +1,39 @@
+"""
+TOTP module — Google Authenticator compatible 2FA codes.
+Uses pyotp (RFC 6238).
+"""
+
+import time
+import pyotp
+
+
+def generate_secret(length: int = 32) -> str:
+    """Generate a new random TOTP secret (base32-encoded)."""
+    return pyotp.random_base32(length=length)
+
+
+def get_code(secret: str) -> str:
+    """Get current 6-digit TOTP code."""
+    return pyotp.TOTP(secret).now()
+
+
+def get_code_with_timer(secret: str) -> dict:
+    """Get current code with timing info for GUI display."""
+    totp = pyotp.TOTP(secret)
+    now = time.time()
+    remaining = 30 - (int(now) % 30)
+    return {
+        "code": totp.now(),
+        "remaining": remaining,
+        "progress": remaining / 30.0,
+    }
+
+
+def verify_code(secret: str, code: str) -> bool:
+    """Verify a TOTP code (with +/- 1 period tolerance)."""
+    return pyotp.TOTP(secret).verify(code, valid_window=1)
+
+
+def format_secret_uri(secret: str, account: str, issuer: str = "ServerManager") -> str:
+    """Generate otpauth:// URI for QR code / authenticator apps."""
+    return pyotp.TOTP(secret).provisioning_uri(name=account, issuer_name=issuer)