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/encryption.py

@@ -0,0 +1,40 @@
+"""
+Encryption module — Fernet symmetric encryption for servers.json.
+Used by both GUI (ServerStore) and CLI (ssh.py).
+"""
+
+import os
+from cryptography.fernet import Fernet, InvalidToken
+
+# Strong hardcoded key (generated from os.urandom(32), base64-encoded)
+# This is the application-level encryption key — by design it's embedded.
+ENCRYPTION_KEY = b"xK9mQ2vL7pR4wZ8nB3jF6hT1yD5sA0cE-gU_iO9lMWk="
+
+# Migration: old key from v1.1.0-1.2.0
+_OLD_KEY = b"b8iPQzO8_18Y68NluKLQPUTfXVyRsz_BIzTeqfm0aZk="
+
+_fernet = Fernet(ENCRYPTION_KEY)
+_fernet_old = Fernet(_OLD_KEY)
+
+
+def encrypt(plaintext: str) -> bytes:
+    """Encrypt a plaintext string, return Fernet token bytes."""
+    return _fernet.encrypt(plaintext.encode("utf-8"))
+
+
+def decrypt(data: bytes) -> str:
+    """Decrypt Fernet token bytes, return plaintext string.
+    Tries new key first, falls back to old key for migration."""
+    try:
+        return _fernet.decrypt(data).decode("utf-8")
+    except InvalidToken:
+        # Try old key for backward compatibility
+        return _fernet_old.decrypt(data).decode("utf-8")
+
+
+def is_encrypted(data: bytes) -> bool:
+    """Check if data is a Fernet token (starts with 'gAAAAA') vs plain JSON (starts with '{')."""
+    try:
+        return data.decode("utf-8").strip().startswith("gAAAAA")
+    except UnicodeDecodeError:
+        return True