About dialog: fix unreadable dark-mode text and missing version

The subtitle and version line used palette(mid), a role meant for 3D
bevel/shadow decoration, not text -- it has poor contrast against the
window background in dark themes. Replaced with a color blended from
the widget's actual text/window palette colors, so it stays readable
(de-emphasized but never low-contrast) in either theme.

Also: QCoreApplication::applicationVersion() was never being set
anywhere, so the dialog always showed "Development build" regardless
of the actual built version. CMake's PROJECT_VERSION is now exposed
to the app via a compile definition and wired into
setApplicationVersion() at startup.

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

CMakeLists.txt

@@ -134,6 +134,7 @@ add_executable(orbithub WIN32 MACOSX_BUNDLE ${ORBITHUB_SOURCES})
 
 target_link_libraries(orbithub PRIVATE Qt6::Widgets Qt6::Sql)
 target_link_libraries(orbithub PRIVATE KodoTerm::KodoTerm)
+target_compile_definitions(orbithub PRIVATE ORBITHUB_VERSION_STRING="${PROJECT_VERSION}")
 if(TARGET freerdp AND TARGET winpr)
     target_compile_definitions(orbithub PRIVATE ORBITHUB_HAS_FREERDP)
     target_include_directories(orbithub PRIVATE