Scope the screenshot ignore rule to the repository root

The previous "*.png" entry in .gitignore matched every PNG file
anywhere in the tree, including the existing screenshots under
docs/ and the default avatar at assets/default-screenshot.png,
and would have silently dropped any future PNG asset added by a
contributor.  Replace it with "/*.png", which only ignores the
ad-hoc Playwright screenshots that land in the repository root
during local front-end development.

Addresses #458 (comment)

Assisted-by: Claude Code:claude-opus-4-7

Author: dahlia

.gitignore

@@ -9,7 +9,7 @@ mise.local.toml
 **/skills/npm-*
 src/public/uno.css
 .playwright-mcp/
-*.png
+/*.png
 
 coverage/**
 !coverage/.gitkeep