Add VNC support (Milestone 6, issue #3)

Implements RFB (RFC 6143) directly against QTcpSocket. No permissively
licensed VNC client library exists to vendor the way FreeRDP was for RDP:
LibVNCClient is GPLv2, gtk-vnc is LGPL but GTK-tied, and vendoring either
would force a licensing decision on the whole (MIT) project. This is
from-scratch protocol code instead, threaded like SshSessionBackend (a
QObject on its own QThread driven by Qt's own async socket signals)
rather than RdpSessionBackend's manual worker-thread/blocking-loop
pattern, since QTcpSocket is already async.

Scope, matching SessionTab's existing SSH/RDP dispatch pattern
(session_backend_factory.cpp, session_tab.cpp's widget construction and
signal wiring) and VncDisplayWidget mirroring RdpDisplayWidget's
scale-to-fit rendering:
- Protocol handshake: RFB 3.3/3.7/3.8 negotiated explicitly (the
  SecurityResult message only exists in 3.8; pre-3.8 servers signal auth
  failure by closing the socket, which the disconnect handler accounts
  for)
- VNC Authentication (DES challenge-response, via OpenSSL's classic DES
  API) and no-auth security types
- Raw + CopyRect framebuffer decoding into a persistent QImage, requesting
  a fixed 32bpp format whose byte layout matches QImage::Format_RGB32
  directly (same zero-conversion trick RdpSessionBackend uses for
  FreeRDP's GDI buffer)
- Keyboard (Qt key -> X11 keysym, including the Unicode-beyond-Latin-1
  keysym convention) and mouse/wheel input forwarding

Explicit non-goals for this pass (see docs/PROGRESS.md for the full
list): Apple's Screen Sharing auth (so this can't yet reach macOS's
built-in VNC server), compression encodings beyond Raw/CopyRect, dynamic
resize, remote cursor shape sync, clipboard sync.

19 unit tests (tests/test_vnc_session_backend.cpp): pure-function
coverage (DES key prep verified against an independently documented test
vector for password "COW", X11 keysym mapping, socket-error mapping) plus
state-machine coverage against a scripted in-process fake RFB server
covering all three protocol-version handshake shapes, auth success/
failure, unsupported security types, and pixel-accurate Raw decoding.
That harness caught a real re-entrancy bug: QAbstractSocket::abort()
synchronously re-emits disconnected() before returning, so
failConnection() calling it was silently letting a second, generic
disconnected-socket handler overwrite an already-correct, specific error
message.

Also verified live against a real, independently implemented VNC server
(TightVNC on Windows): connect with VNC Authentication, correct
framebuffer dimensions and pixel data, clean disconnect, reconnect.

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

CMakeLists.txt

@@ -12,7 +12,8 @@ set(CMAKE_AUTORCC ON)
 
 include(GNUInstallDirs)
 
-find_package(Qt6 6.2 REQUIRED COMPONENTS Widgets Sql)
+find_package(Qt6 6.2 REQUIRED COMPONENTS Widgets Sql Network)
+find_package(OpenSSL REQUIRED)
 
 qt_standard_project_setup()
 
@@ -115,6 +116,8 @@ set(ORBITHUB_SOURCES
     src/session_tab.h
     src/rdp_display_widget.cpp
     src/rdp_display_widget.h
+    src/vnc_display_widget.cpp
+    src/vnc_display_widget.h
     src/terminal_view.cpp
     src/terminal_view.h
     src/session_window.cpp
@@ -123,6 +126,8 @@ set(ORBITHUB_SOURCES
     src/rdp_session_backend.h
     src/ssh_session_backend.cpp
     src/ssh_session_backend.h
+    src/vnc_session_backend.cpp
+    src/vnc_session_backend.h
     src/unsupported_session_backend.cpp
     src/unsupported_session_backend.h
 )
@@ -145,8 +150,9 @@ endif()
 
 add_executable(orbithub WIN32 MACOSX_BUNDLE ${ORBITHUB_SOURCES})
 
-target_link_libraries(orbithub PRIVATE Qt6::Widgets Qt6::Sql)
+target_link_libraries(orbithub PRIVATE Qt6::Widgets Qt6::Sql Qt6::Network)
 target_link_libraries(orbithub PRIVATE KodoTerm::KodoTerm)
+target_link_libraries(orbithub PRIVATE OpenSSL::Crypto)
 target_compile_definitions(orbithub PRIVATE ORBITHUB_VERSION_STRING="${PROJECT_VERSION}")
 if(TARGET freerdp AND TARGET winpr)
     target_compile_definitions(orbithub PRIVATE ORBITHUB_HAS_FREERDP)