Document Gitea authentication setup

docs/GITEA_AUTH.md

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+# Gitea authentication
+
+Remote:
+
+```text
+https://git.noveria.net/bao/genshin-assistant.git
+```
+
+Personal access tokens can be managed here:
+
+```text
+https://git.noveria.net/user/settings/applications
+```
+
+Use the token as the HTTPS password when Git asks for credentials. Do not save
+tokens, passwords, or generated credentials in this repository.
+
+On Windows, this repository uses Git Credential Manager from the system Git
+config:
+
+```powershell
+git config --show-origin --get-all credential.helper
+```
+
+If authentication breaks again, refresh the saved credential in Windows
+Credential Manager or let Git Credential Manager prompt again during:
+
+```powershell
+git push origin main
+```