Improve and complete German translations

[DelayLama]

This PR significantly improves the German localization.

Changes:

• Added missing German translations across all locale namespaces
• Replaced untranslated English strings
• Improved existing German translations for consistency
• Standardized terminology and informal "Du" addressing
• Preserved all i18next placeholders, URLs and markup
• Verified JSON validity and key parity with the English locale

The German locale now has full key coverage compared to the current English locale.

Related to #145

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

Since your pull request originates from a forked repository, GitGuardian is not able to associate the secrets uncovered with secret incidents on your GitGuardian dashboard.
Skipping this check run and merging your pull request will create secret incidents on your GitGuardian dashboard.

🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
-	-	Username Password	35b9d86	public/locales/de/auth.json	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secret safely. Learn here the best practices.
3. Revoke and rotate this secret.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

---

^{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}

[DelayLama]

I've double checked my pull request and all included JSON files contributing to the German translation.

This GitGuardian finding is a false positive. public/locales/de/auth.json is a UI translation file — the flagged lines are German labels for form fields ("yourUsername": "Dein Benutzername", "newPassword": "Neues Passwort"), mirroring the same keys in en/auth.json. No credential is present anywhere in this PR; the generic username/password detector is matching on the key names.

[DelayLama]

This GitGuardian finding is a false positive. public/locales/de/auth.json is a UI translation file — the flagged lines are German labels for form fields ("yourUsername": "Dein Benutzername", "newPassword": "Neues Passwort"), mirroring the same keys in en/auth.json. No credential is present anywhere in this PR; the generic username/password detector is matching on the key names.

[DelayLama]

This GitGuardian finding is a false positive. public/locales/de/auth.json is a UI translation file — the flagged lines are German labels for form fields ("yourUsername": "Dein Benutzername", "newPassword": "Neues Passwort"), mirroring the same keys in en/auth.json. No credential is present anywhere in this PR; the generic username/password detector is matching on the key names.

[meauxt]

Sounds good!

[meauxt]

sorry seems it conflicts with the Crowdin translation that i just merged. If you can resolve the conflict would be great; otherwise, I will try to pull your branch this weekend and do it

[meauxt]

got sometime fix the conflict #242