macOS: build a proper .app bundle with an embedded icon

Same root cause as the earlier Windows exe icon fix: add_executable()
never set MACOSX_BUNDLE, so on macOS this produced a bare Mach-O binary
rather than a proper .app bundle -- and without a bundle there's no
Info.plist/.icns mechanism for Finder to show a custom icon at all.

Added packaging/macos/orbithub.icns, rendered at up to 1024px directly
from the same createOrbitHubAppIcon() logic used at runtime (via a
one-off export tool, not committed) so it matches what the app actually
looks like. CMake's MACOSX_BUNDLE keyword and MACOSX_BUNDLE_ICON_FILE
wire it into an auto-generated Info.plist; both keywords are no-ops on
other platforms. The profile database uses QStandardPaths::AppDataLocation,
not the executable's own path, so this doesn't affect where existing
profiles are found.

Build output on macOS changes from build/orbithub to build/orbithub.app
(launch with `open build/orbithub.app`); docs/BUILDING.md updated to match.

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

CMakeLists.txt

@@ -115,7 +115,19 @@ if(WIN32)
     list(APPEND ORBITHUB_SOURCES packaging/windows/orbithub.rc)
 endif()
 
-add_executable(orbithub WIN32 ${ORBITHUB_SOURCES})
+if(APPLE)
+    list(APPEND ORBITHUB_SOURCES packaging/macos/orbithub.icns)
+    set_source_files_properties(packaging/macos/orbithub.icns PROPERTIES
+        MACOSX_PACKAGE_LOCATION "Resources"
+    )
+    set(MACOSX_BUNDLE_ICON_FILE orbithub.icns)
+    set(MACOSX_BUNDLE_BUNDLE_NAME "OrbitHub")
+    set(MACOSX_BUNDLE_GUI_IDENTIFIER "io.orbithub.OrbitHub")
+    set(MACOSX_BUNDLE_BUNDLE_VERSION "${PROJECT_VERSION}")
+    set(MACOSX_BUNDLE_SHORT_VERSION_STRING "${PROJECT_VERSION}")
+endif()
+
+add_executable(orbithub WIN32 MACOSX_BUNDLE ${ORBITHUB_SOURCES})
 
 target_link_libraries(orbithub PRIVATE Qt6::Widgets Qt6::Sql)
 target_link_libraries(orbithub PRIVATE KodoTerm::KodoTerm)