GUI: deck import from MTG Arena (MTGA) must be improved

https://magicarena.fandom.com/wiki/Deck_Import

It must support new words:
* `Commander` to start sideboard section
* `Deck` to start deck section
* Current version support only `empty line` as switch from deck to sideboard;

Example deck:
```
Commander
1 Alela, Artful Provocateur (ELD) 324

Deck
1 Temple of Silence (M20) 256
1 Watery Grave (GRN) 259
6 Plains (ELD) 251
```

No source file or test is named. Start by searching the Java codebase for the MTG Arena deck importer and existing handling of empty lines, then compare it with the supplied Commander and Deck example. Done means both section labels switch parsing to the appropriate section while existing imports continue to work.

Written by the indexing model from the issue text.