refactor: split frontend.c into domain-grouped modules

[deevus]

Summary

• Split the monolithic frontend.c (7,068 lines) into 10 focused source files grouped by domain
• Added missing extern declarations to frontend.h for symbols that were previously file-local (szSelectEng, szConfigEng, font*_ascii_br, fade_redraw_bg)
• Consolidated PREVIEW_* viewport defines into frontend.h instead of duplicating across files
• Added menu_render*.c files to CMake, Make, and MSVC build systems (were only in build.zig)
• Reverted font*_ascii types back to char[256] in header to match all function signatures

New file layout

File	Lines	Content
frontend_data.c	534	Global variables and data tables
frontend_screens.c	1093	Main menu flow (title, copy, select_screen)
frontend_select_disk.c	377	Disk/save selection
frontend_select_car.c	584	Car selection
frontend_select_players.c	412	Player setup
frontend_select_type.c	649	Game type selection
frontend_select_track.c	396	Track selection
frontend_config.c	2528	Configuration menu
frontend_network.c	462	Network lobby and restart
frontend_util.c	303	Params, cheat names, name validation

Test plan

• Clean zig build passes with zero errors
• Verify no regressions in game frontend menus
• Verify Windows build (MSVC .vcxproj)