Extract shared board rendering service

[francinv]

GameBoardRenderer and SetupBoardRenderer duplicate tile geometry, grid drawing, and piece rendering logic. BoardCoordinateMapper exists only in setup — game reimplements the same row-flip math internally.

What to do:

• Extract common board rendering (grid, tiles, pieces) into a shared base class or utility
• Move BoardCoordinateMapper out of screens/setup/ into a shared location (e.g. utils/)
• Both renderers should extend/use the shared component

Key files:

• GameBoardRenderer.java:55-99, 101-169
• SetupBoardRenderer.java:42-60, 78-139
• BoardCoordinateMapper.java:11-42 (setup only)
• GameBoardRenderer.java:64-72 (reimplements coordinate mapping)