feat: add tutorial and onboarding experience

[nikolareljin]

Summary

• add a guided in-game tutorial with contextual hints for movement, combat, city building, diplomacy, and end-turn flow
• add skip and replay support (Game Menu -> Tutorial & Onboarding)
• persist tutorial progress in save data and add legacy-save fallback behavior
• update UI styling and release docs/versioning for 1.13.0

Validation

• node --check js/ui.js
• node --check js/state.js
• node --check js/map.js
• ./scripts/check_release_version.sh

Closes #24

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Enforce end-turn requirement before finishing tutorial

At the last onboarding step, clicking Finish calls completeTutorial() directly, which bypasses the required-action guard in advanceTutorialStep. In practice, players can mark the tutorial as completed without ever performing the Step 5 endTurn action, so onboarding state is persisted as done while the final mechanic was never demonstrated.

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