chore: add web/img/*.png to gitignore exception for microplastic research screenshots

5 research source screenshots from ScienceDaily, WHO, NEJM, PNAS, 科普中国

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.gitignore

@@ -48,6 +48,7 @@ worktrees/
 *.png
 !docs/**/*.png
 !cfc-web/public/logo.png
+!web/img/*.png
 
 # Python
 *.py