Add widescreen width CLI override

[mstan]

Summary:

• Adds --widescreen-width and NDS_WIDESCREEN_WIDTH for fixed Widescreen widths.
• Restricts exposed fixed widths to OpenGL-compute-safe values: 256, 320, 384, and 448.
• Reuses the frontend config parser so TOML, CLI, and env validation match.
• Enables launchers to offer Dynamic Widescreen plus fixed width choices without crash-on-launch values.

Validation:

• frontend_config_test.exe passed after rebuild.
• nds_runner.exe rebuilt in the combined validation branch.

Bead: beads-lqa.31