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/session_tab.cpp

@@ -998,6 +998,10 @@ void SessionTab::requestConnectOptions(
 {
     SessionConnectOptions baseOptions;
     baseOptions.knownHostsPolicy = m_profile.knownHostsPolicy;
+    // The backend's own Profile copy was captured when it was constructed
+    // and never sees later edits to m_profile (e.g. the username prompt
+    // below) -- it has to travel through here instead.
+    baseOptions.username = m_profile.username.trimmed();
 
     const bool isSsh = m_profile.protocol.compare(QStringLiteral("SSH"), Qt::CaseInsensitive) == 0;
     const bool isRdp = m_profile.protocol.compare(QStringLiteral("RDP"), Qt::CaseInsensitive) == 0;