Add screenshot capture script for documentation

- Capture main UI pages (Encode, Decode, Generate, Tools, About)
- Capture auth pages (Login, Setup, Account, Recover)
- Auto-convert PNG to WebP for smaller file sizes
- Update .gitignore to track this script

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -69,6 +69,7 @@ scripts/*
 !scripts/validate-release.sh
 !scripts/smoke-test.sh
 !scripts/setup-trusted-certs.sh
+!scripts/screenshots.sh
 
 # Web UI auth database and SSL certs
 instance/