fix: normalize fallback version (#2386)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Updated the fallback version to “0.0.0” when build metadata isn’t
available, replacing the previous “Unknown” label. This provides a
clear, semantic version in apps, logs, and package info.
* Improves consistency across environments by preventing ambiguous
“Unknown” version displays. No functional changes to features; only the
reported version string is affected when source control metadata cannot
be detected.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: njzjz

pyproject.toml

@@ -109,7 +109,7 @@ cmake.version = ">=3.26"
 
 [tool.setuptools_scm]
 write_to = "reacnetgenerator/_version2.py"
-fallback_version = "Unknown"
+fallback_version = "0.0.0"
 
 [tool.cibuildwheel]
 test-command = [