Add legacy .elp support

[erseco]

This pull request adds support for legacy .elp files in addition to the existing .zip and .elpx formats. Users can now upload, drag-and-drop, or load .elp files, which will be processed using a new converter module. The UI, validation logic, and error handling have all been updated to reflect this expanded support, with appropriate internationalization changes for both English and Spanish.

Legacy .elp file support:

• Added a new processLegacyElp function in js/app.js to handle .elp files using the ElpConverter module, including error handling and integration with the Service Worker.
• Injected required scripts for legacy .elp support in index.html, including polyfills and the converter module.

UI and validation updates:

• Updated drag-and-drop instructions, file input acceptance, and error messages in index.html and lang/en.json/lang/es.json to include .elp files. [1] [2] [3]
• Modified file validation logic in js/app.js to recognize .elp files for both local and URL-based uploads. [1] [2]

Internationalization:

• Added new loading and error messages for .elp conversion in both English and Spanish locales (lang/en.json, lang/es.json). [1] [2] [3] [4]