Keep imported server.properties managed by server.env

[cotore-game]

Changes

• Convert supported imported properties into server.env and extra keys into CUSTOM_SERVER_PROPERTIES; keep overrides enabled.
• Reject incompatible world names, ports, and unsupported syntax before changing server data.
• Move the import action under Server settings and update English/Japanese guidance.

Verification

• bash tests/run-tests.sh: 134 passed, 1 skipped (zip unavailable locally)
• bash -n and git diff --check