Add mkcert support for browser-trusted HTTPS certificates

No more browser warnings! mkcert creates locally-trusted certs.

Pi Setup:
- Auto-install mkcert during setup
- Generate trusted certs when HTTPS enabled
- Copy CA to /static/ca/rootCA.pem for easy device setup
- New devices can download CA via HTTP and install it

Docker:
- docker-entrypoint.sh checks for mkcert, falls back to openssl
- Shows instructions for CA distribution to other devices

Scripts:
- Added setup-trusted-certs.sh helper for local dev
- Installs mkcert, generates certs, shows device setup instructions

To trust on new devices:
1. Download: http://stegasoo.local/static/ca/rootCA.pem
2. Install as trusted CA in browser/OS

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitignore

@@ -68,6 +68,7 @@ test_data/*.png
 scripts/*
 !scripts/validate-release.sh
 !scripts/smoke-test.sh
+!scripts/setup-trusted-certs.sh
 
 # Web UI auth database and SSL certs
 instance/