feat: import server.properties and open server data in Explorer

[cotore-game]

Changes

• Import a distributed server.properties into a stopped server, preserving a timestamped backup of the previous file.
• Make data/server.properties the source of truth only for imported servers; keep the existing server.env mode for other servers.
• Let the server settings TUI edit imported properties directly while whitelist and OP member lists remain in server.env.
• Add Windows file selection and Explorer shortcuts for the server and persistent data folders.
• Document the workflow in English and Japanese.

Verification

• 128 specification tests passed locally in WSL2; one nested ZIP test was skipped because zip is not installed there.
• Locale validation, Bash syntax checks, and git diff --check passed.
• Windows file picker and Explorer launching are covered by mocked tests, but have not yet been exercised manually on the target WSL2 desktop.

The release/v1.1.2 base branch was created from main and contains VERSION=1.1.2 plus CI coverage for release-targeted PRs. This PR does not tag or publish a release.