Fix connect-time username prompt never reaching SSH/RDP authentication

The username entered at the connect-time prompt (added for issue #21)
was updating SessionTab's own in-memory Profile copy, but
SshSessionBackend/RdpSessionBackend are constructed with -- and only
ever read from -- their own separate Profile copy on a worker thread,
which never saw that edit. Authentication was still built from the
original (blank) username regardless of what was typed into the
prompt.

SessionConnectOptions gains a username field, populated by SessionTab
on every connect attempt and threaded through the same way password
already is; both backends now prefer options.username over
profile().username. Covered by a new SSH regression test using an
exact-match fixture host that only succeeds for a specific
user@host target.

Bump version to v2026.9.16.5.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/rdp_session_backend.cpp

@@ -1640,7 +1640,13 @@ void RdpSessionBackend::workerMain()
     const Profile& p = profile();
 
     const QString host = p.host.trimmed();
-    QString username = p.username.trimmed();
+    // m_activeOptions.username carries a value prompted for at connect time
+    // (see SessionTab::requestConnectOptions()) when the saved profile's
+    // own username was blank; profile().username never sees that edit
+    // since this backend's Profile copy was captured at construction time.
+    QString username = m_activeOptions.username.trimmed().isEmpty()
+        ? p.username.trimmed()
+        : m_activeOptions.username.trimmed();
     QString domain = p.domain.trimmed();
     if (domain.isEmpty()) {
         const int domainSeparator = username.indexOf(QLatin1Char('\\'));