test(lang-en): add comprehensive unit tests for English translation labels

[kanishka-2007-tech]

Description

Adds a dedicated test suite for English localization labels defined in lib/i18n/badgeLabels.ts.

This ensures the English translation mapping remains stable and prevents regressions in badge rendering and localization behavior.

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Changes

Added

• lib/i18n/languages/en.test.ts

Test Coverage

1. Verifies the en language key exists in the labels dictionary.
2. Verifies getLabels('en') returns the expected English translation object.
3. Verifies all required translation properties are defined and non-empty:
   • CURRENT_STREAK
   • ANNUAL_SYNC_TOTAL
   • PEAK_STREAK
   • COMMITS_THIS_MONTH
   • VS_LAST_MONTH
4. Verifies the exact English translation values match the source definitions.
5. Verifies unsupported languages correctly fall back to English translations.

Fixes #2331

Pillar

• 📐 Pillar 2 — Geometric SVG Improvement
• 🛠️ Other (Bug fix, refactoring, docs)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• I have updated README.md if I added a new theme or URL parameter.
• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard (no raw elements, smooth animations, correct fonts).
• (Recommended) I joined the CommitPulse Discord community for contributor discussions, mentorship, and faster PR support.

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