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>

docs/BUILDING.md

@@ -35,7 +35,7 @@ cmake -S . -B build -G Ninja \
   -DCMAKE_BUILD_TYPE=Release \
   -DCMAKE_PREFIX_PATH="$(brew --prefix qt@6);$(brew --prefix openssl@3)"
 cmake --build build
-./build/orbithub
+open build/orbithub.app
 ```
 
 ## Windows 11 (PowerShell + MSVC + vcpkg)
@@ -82,7 +82,8 @@ If you already have `Qt 6` from the Qt installer and do not want vcpkg Qt, you c
 - OrbitHub builds vendored `KodoTerm`, `libvterm`, and `FreeRDP` from `third_party/`.
 - If Qt is installed in a custom location, pass `-DCMAKE_PREFIX_PATH=/path/to/Qt/6.x.x/<toolchain>` to CMake.
 - Build output executable:
-  - Linux/macOS: `build/orbithub`
+  - Linux: `build/orbithub`
+  - macOS: `build/orbithub.app` (a proper app bundle, launch with `open build/orbithub.app`)
   - Windows: `build\\orbithub.exe`
 
 ## Linux Packaging