docs: refresh stable installation and development guidance

[gdarko]

Summary

• identify these as the supported 2.x installation docs and point downloads at the actual download page
• correct production Compose choices, public URL settings, database host, and SQLite path
• align manual requirements with Laravel 13 and PHP 8.4
• replace obsolete .dev and npm instructions with the 2.x ./devenv, Node 24, and pnpm workflow
• correct deployment, mobile-app, and Crowdin wording

Validation

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