Untrack standalone/src-tauri/gen/ and add to .gitignore

These files are generated by cargo check/build on each platform, so
tracking them causes churn and platform-dependent diffs. Files remain
on disk; only removed from git.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: nedtwigg

.gitignore

@@ -21,6 +21,7 @@ release-signed/
 # Tauri / Standalone
 standalone/src-tauri/target/
 standalone/src-tauri/binaries/
+standalone/src-tauri/gen/
 standalone/dist/
 standalone/sidecar/node_modules/
 standalone/node_modules/