Replace custom-game overlay and fix bot packet decoding

[Atchbo2001]

Root cause

The custom-game screen was incorrectly implemented as a React/browser overlay instead of using Sauerbraten's native GUI. Separately, AI clients start at client number 128, but N_POS was decoded with Sauerbraten's signed compact-integer format. At CN 128+, that consumed the wrong number of bytes and shifted the shared packet stream, producing the reported malformed N_SHOOT, edit-message, movement, targeting, and death behavior.

Fixed

• Removed the React custom-game overlay and browser password prompt.
• Added native Sauerbraten newgui screens for custom-game setup, game-mode selection, map selection, and password entry.
• Added the complete built-in map catalog: all FFA, capture, CTF, concept, single-player, and RPG map groups.
• Added discovery of extra installed packages/base/*.ogz maps.
• Kept public, password-protected, and unlisted custom games, bot count, bot skill, and round-duration controls.
• Corrected N_POS to encode/decode client numbers with PutUint/GetUint.
• Added a regression test at bot client number 128 followed by another protocol message, verifying the packet boundary remains intact.

Validation

GitHub Actions run 29976339603 completed successfully:

• Targeted protocol regression test passed before the full build.
• Game assets built.
• WebAssembly engine built.
• Web client and native-menu wiring built.
• Go multiplayer backend built.
• Linux server binary verified.
• Integrated HTTP and WebSocket smoke test passed.
• Flat Pterodactyl package assembled and uploaded.
• Downloaded artifact SHA-256 matched GitHub's recorded digest.
• Inner deployment ZIP passed a complete archive integrity test.