Make the build App Store-ready

[EitaroY]

• Enable App Sandbox for every build (ENABLE_APP_SANDBOX). Window control keeps working because macOS allows sandboxed apps to use the Accessibility API once the user grants permission — the same configuration as the established window managers on the store. Keeping local builds sandboxed too means the OSS build and the store build behave identically.
• Add PrivacyInfo.xcprivacy: no tracking, no data collection, UserDefaults declared with reason CA92.1.
• Set ITSAppUsesNonExemptEncryption=NO so uploads skip the export compliance question, and bump MARKETING_VERSION to 1.0.
• install.sh: preserve entitlements when re-signing with the stable certificate — a plain codesign --force would silently strip the sandbox and diverge from the store build.
• docs/APPSTORE.ja.md: full release runbook (ASC setup, app record, pricing, store listing drafts in JA/EN, keywords, review notes template, pre-submission checklist). Update DESIGN.ja.md §7/§11 and BUILDING.md for the sandbox change.

Verified: swift test passes, sandboxed Release build launches and creates its container, entitlements and Info.plist keys confirmed in the built bundle, xcodebuild archive succeeds.

What & why

Checklist

• swift test passes
• The app builds and runs (⌘R)
• Ran the relevant items of the manual smoke-test checklist if Windowing/ or Input/ changed
• Updated the docs (README.md / README.ja.md / docs/DESIGN.ja.md) if behavior or architecture changed
• No new third-party dependencies