feature: Enable a configurable worldwide leaderboard

[O2sa]

The leaderboard currently focuses on country-specific rankings. We should add support for a worldwide leaderboard that aggregates users globally and limits the number of displayed users through a runtime environment variable.

Tasks

• Add support for a worldwide leaderboard route/state in the leaderboard data flow.
• Query users across all countries when the selected leaderboard slug is worldwide.
• Add a runtime environment variable (e.g., WORLDWIDE_LEADERBOARD_DISPLAY_LIMIT) to control how many users are shown.
• Apply a sensible fallback limit when the runtime variable is missing or invalid.
• Update .env.example and relevant documentation with the new variable.
• Verify the UI handles the worldwide leaderboard title, count, and ranking correctly.

Tip

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