chore: stop tracking .DS_Store

Add .DS_Store to .gitignore and untrack the two committed copies
(.DS_Store at the root and .claude/.DS_Store). macOS noise that
shouldn't have been versioned in the first place.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,4 @@
+.DS_Store
 /.phpunit.cache
 /node_modules
 /public/build