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>

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.21)
 
-project(OrbitHub VERSION 2026.9.16.4 LANGUAGES CXX)
+project(OrbitHub VERSION 2026.9.16.5 LANGUAGES CXX)
 
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)