[laugmanuel]

Allow additional request headers per account, for example the credentials of a pre-authentication proxy

This fixes #5821

• Login: collapsible "Advanced options" below the QR button with a "Name: value" field. Headers are captured only for server checks the user started, registered provisionally for the typed origin and stored in the account only if the account ends up on that origin.
• Accounts: headers are kept as account user data, registered at app start, reconciled after an account was removed and editable later via "Custom headers" in the account menu.
• Web views: GET requests to a registered origin go through the library client, which follows redirects only within that origin and syncs cookies on every hop; main frame redirects navigate to their target. Other requests of the page's fetch and XMLHttpRequest calls get the headers from a per-origin page script; fetch rejects redirects for them.
• Glide: images of registered origins are loaded through the library client.
• The transport for web views and Glide verifies host names like the web view does, accepting a mismatch only for certificates the user accepted.

This PR requires nextcloud/android-library#2192 and a library bump.

🖼️ Screenshots

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using Claude Code with Claude Opus 5.5

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