Feat/stats dashboard

[kbtale]

Summary

Adds a comprehensive stats and analytics system (StatsModal), practice telemetry tracking, achievement progression engine, 30-day activity tracking, collapsible sidebar categories with persistent state, header controls refinements, and guide cleanup across all supported locales.

Key Changes

Stats Modal (StatsModal): Accessible modal dialog accessible via a dedicated header button next to Settings, providing overall mastery score, category mastery breakdowns, daily activity tracking over the last 30 days, and real-time achievement progress.
Telemetry and Storage Engine: Extended storageService with methods for recording session telemetry (polymath_telemetry), daily practice counts (polymath_activity), unlocked achievements (polymath_achievements), and collapsed category state (polymath_collapsed_categories).
Achievement System: Implemented domain logic in src/modules/stats/logic.ts to compute progress across streak milestones, module mastery, category completion, and polymath achievements, supported by non-intrusive AchievementToast alerts.
Sidebar & Header Improvements: Added category accordion collapse/expand with localStorage persistence, eliminated spacing gaps between modules in the left panel, added a right-aligned language selector dropdown with the languages icon, and integrated the stats button alongside settings.

Type of Change

• Bug fix
• New feature
• Refactoring
• Performance improvement
• Accessibility improvement
• Documentation update

Checklist

• npm run check passes (Biome lint & format)
• npm run test passes (all unit tests)
• npm run build passes (TypeScript strict mode)
• New i18n keys added to all 3 locales (en, es, it)
• No inline styles added (use SCSS modules)