Prepare frontend translations and markdown placeholder option

[martindell]

Summary

• Move frontend Entity Notes strings out of entity-notes.js and into translation JSON files
• Load frontend translation strings from custom_components/entity_notes/translations/*.json
• Add a Display option to hide the long Markdown placeholder text while keeping the default behavior unchanged
• Update translation contribution docs

Testing

• python3 -m json.tool custom_components/entity_notes/translations/en.json >/dev/null
• python3 -m compileall custom_components/entity_notes
• JS parse check after runtime placeholder substitution
• git diff --check
• Local Home Assistant deploy/test via ha-deploy-dev.sh