Add international language levels (A1-C2) to skills system

[Copilot]

This PR replaces the generic skill levels (Beginner, Intermediate, Advanced, Expert, Master) with internationally recognized language proficiency levels based on the Common European Framework of Reference for Languages (CEFR).

Changes

New Language Levels:

• A1 - Basic User (Breakthrough)
• A2 - Basic User (Waystage)
• B1 - Independent User (Threshold)
• B2 - Independent User (Vantage)
• C1 - Proficient User (Effective Operational)
• C2 - Proficient User (Mastery)

Technical Implementation:

• Updated SkillLevel TypeScript type to use A1-C2 levels
• Modified slider components to support 6 levels instead of 5 (maxValue changed from 4 to 5)
• Updated default skill level from "Intermediate" to "B1"
• Updated site configuration with equivalent mappings for existing skills
• Fixed all test suites to use the new language level system

Benefits:

• Provides standardized, internationally recognized proficiency indicators
• Makes it easier for portfolio visitors to understand actual skill levels
• Maintains consistent skill assessment across different languages and technical skills
• Improves professional credibility by using established frameworks

The implementation maintains backward compatibility with existing data structures while providing more meaningful skill level indicators that are widely understood in professional and academic contexts.

Fixes #66.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• download.cypress.io
  • Triggering command: node index.js --exec install (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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