Nitro V3 update: CLI-only (Linux), settings via .env

- Moved Nitro V3 update from web UI (Commandocentrum) to CLI-only
- Removed configure paths form and runUpdateNitrov3() from admin panel
- update-Nitrov3.sh now loads .env automatically from its directory
- Added all NITRO_* env vars to .env.example.linux and .env
- Removed configurable paths from database (replaced by .env)
- Updated README and CHANGELOG

CHANGELOG.md

@@ -23,6 +23,9 @@
 - XAMPP support removed (security warning added)
 - `.env.example` → `.env.install` with step-by-step guide
 - `.env.standard` → `.env.example.linux` + `.env.example.windows`
+- Nitro V3 update moved from web UI to CLI (Linux-only)
+- Nitro V3 settings moved from database to `.env` file
+- `update-Nitrov3.sh` now loads `.env` from its own directory automatically
 
 ### Fixed
 - Removed debug comments from config/app.php