fix(frontend): generate and validate maps before Docker builds

[WilliamKwanProgramming]

Closes #19

Assigned by @leocagli: #19 (comment)

Fixes clean Docker builds shipping without map assets. Generates maps in a separate Docker stage with the unchanged canonical exporter, preserves server/mapas_source for NPC loading, and validates every source map ID before the frontend build. Adds README instructions, regression tests, and final-image map validation in CI.

Rebased onto upstream main 0683f86. Five map-validation tests and the new missing-terrain exporter regression pass locally; all 294 real maps export and validate. The missing-terrain test verifies exit 1 against a successful two-map control.

All five fresh CI jobs passed (protocol, API, server, frontend, Docker), including final-image map validation. Secret Scan also passed.

CI: https://github.com/WilliamKwanProgramming/OpenAO/actions/runs/35150445334

Docker is unavailable locally; Docker validation runs in CI. No interactive game-rendering test is claimed. Prepared with AI assistance. Payment, if awarded, is handled through GrantFox.

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[WilliamKwanProgramming]

@leocagli PR #375 is submitted within the assignment window. GitHub is currently holding the repository CI/Secret Scan with “workflows awaiting approval” for this fork PR. The same commit already passed the full CI and Secret Scan on my fork, including Docker/final-image map validation. When convenient, could you approve the upstream workflows so the required checks can run? Thanks.