[ClausValcaTD]

Fixes blank/corrupted UI screens on NES and Game Boy targets:

1. NES (targets/nes/src/ui_nes.c & main.c): Latch and write Palette RAM at $3F00 with background palette colors ($0F, $30, $16, $27) and fill remaining entries. Clean PPU scroll reset and PPU_MASK=0x1E setup.
2. Game Boy (targets/gameboy/src/ui_gb.c & main.c & Makefile): Initialize GBDK font system (font_init, font_color(3, 0), font_load(font_min), font_set), enable SHOW_BKG and DISPLAY_ON, set BGP_REG = 0xE4 and CGB Palette 0, and use -Wm-yC CGB build flag.
3. Automated verification: Updated scripts/capture_screenshots.sh and regenerated verified screenshots/nes_screenshot.png and screenshots/gb_screenshot.png with image variance > 50.0.

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PR created automatically by Jules for task 8251907147048057832 started by @ClausValcaTD

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