* Fix GIT_COMMIT_HASH not set in Flatpak builds
The env var check was gated inside the .git directory check, so
Flatpak builds (which exclude .git from the sandbox) always fell
back to "0000000". Lift the env var check to top level and inject
the commit hash into the Flatpak manifest via build-options.env.
* Switch Flatpak build to Clang/LLD via LLVM 21 SDK extension
- Add org.freedesktop.Sdk.Extension.llvm21
- Bump runtime to GNOME 49 (SDK 25.08) for llvm21 availability
* fix build errors and improving build speed for flatpak
* fxi more build errors
* Update error messages for GNOME Platform and SDK versions
