Correct documented dev ports and ignore local tooling output

CLAUDE.md described the backend on 8000 and the frontend on 3000, but
frontend/.env has overridden these to 8002 and 5172 (matching .env.example
and the CORS defaults in settings.py). Following the documented ports leads
to checking servers that are not there.

Also records the CORS_ALLOWED_ORIGINS override and the real default origin
list, and notes that the frontend port comes from frontend/.env. The serial
bridge's BACKEND_URL is left at 8000 because that is what serial_bridge/.env
actually contains - it points at a port Django no longer listens on, which
is a config issue to fix separately, not a documentation error.

Adds .claude/screenshots/ and .playwright-mcp/ to .gitignore so local
verification output stays out of the repository.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -73,3 +73,7 @@ tools/license_private.key
 
 # Local config backups
 *.env.bak
+
+# Local tooling artifacts
+.claude/screenshots/
+.playwright-mcp/