feat(i18n): add German (de) language to the UI language switcher

[xrvnd]

Description

Fixes #5330

Adds German (de) as a fully supported UI language. Users with their browser set to German, or who select it from the navbar dropdown, will see the entire interface translated — navbar, footer, landing page, customization studio, and dashboard — with automatic fallback to English for any missing key.

Files changed:

locales/de.json — new file, full German translation of all ~200 keys across every section
context/TranslationContext.tsx — added de to the Language union, translations map, and LANGUAGE_LABELS ('Deutsch')
locales/de.test.ts — new parity test asserting exact key completeness vs en.json, no empty values, and {{placeholder}} token integrity

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Bug fix, refactoring, docs)

Visual Preview

(Language switcher --> Deutsch now appears in the navbar dropdown)

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.

[vercel]

@xrvnd is attempting to deploy a commit to the jhasourav07's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Aamod-Dev]

Thanks for the contribution! The implementation looks solid and addresses the issue well. Approving.

[xrvnd]

@Aamod007 please use appropriate tags for this issue and merge
thanks!

[github-actions]

🎉 Congratulations @xrvnd! Your PR has been successfully merged. 🚀

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