Language-specific and country-language developer rankings

[sajeetharan]

Problem

The leaderboard exposes an overall position, and #25 covers country/city leaderboards, but developers cannot answer more personally meaningful questions such as "What is my rank among TypeScript developers?" or "Am I the top Rust developer in Sri Lanka?"

Proposed solution

Add language-specific and country + language leaderboard scopes using the existing DevGlobe score.

Acceptance criteria

• Add a language leaderboard for each topLanguage
• Show a developer's rank and population, for example #12 of 4,820 TypeScript developers
• When country data is available, show the combined rank, for example #1 TypeScript developer in Sri Lanka
• Add language and country + language filters to the leaderboard UI
• Display the scoped rank on developer details and shareable profile/card surfaces
• Define deterministic tie handling and exclude developers with missing scope data from the denominator
• Show the score basis and data update date near each rank
• Add tests for rank, population, ties, and missing country/language values

Technical notes

• Reuse the score calculated in lib/scoring.js; this feature changes the comparison cohort, not the scoring formula.
• Prefer precomputed rank fields or a cached aggregate for common cohorts rather than scanning all developers per profile request.
• Country normalization can build on Country and city leaderboards #25.

Related issues

• Country and city leaderboards #25 covers country and city leaderboards, but not language cohorts.
• Badges and milestones system #26 covers achievement badges and can consume these ranks later.
• Shareable developer profile URLs with OpenGraph meta tags #20 and Feature: Public shareable developer profiles #50 cover shareable profile surfaces.