Store password hash instead of plaintext for login persistence

Save SHA1(UPPER(user):UPPER(pass)) hash to login.cfg instead of the
plaintext password. On subsequent logins, use the stored hash directly
with a new authenticateWithHash() method that bypasses password hashing.
The password field shows a placeholder when using a stored hash.

include/auth/auth_handler.hpp

@@ -43,6 +43,7 @@ public:
 
     // Authentication
     void authenticate(const std::string& username, const std::string& password);
+    void authenticateWithHash(const std::string& username, const std::vector<uint8_t>& authHash);
 
     // Realm list
     void requestRealmList();