Update Kaneo to v2.6.0

[NI-R0]

Pull Request Title: Update Kaneo compose.yaml to v2.6.0

Description

Kaneo's configuration files were outdated and did not work with the latest official images.
This PR updates the compose.yaml and env file to match Kaneo's latest release.

Related Issues

• Fixes 🏗️ New Service: Kaneo #251

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring

How Has This Been Tested?

1. Deployed the stack on my Ubuntu server using docker compose up -d
2. Confirmed Tailscale container joined the Tailnet successfully
3. Accessed the WebUI via MagicDNS URL on a mobile device over cellular and Wifi
4. Confirmend API and WebUI work together as intended by inspecting Firefox's developer tools network tab

Checklist

• I have performed a self-review of my code
• I have added tests that prove my fix or feature works
• I have updated necessary documentation (e.g. frontpage README.md)
• Any dependent changes have been merged and published in downstream modules

Additional Notes

In order for the frontend to be able to communicate with the backend, I added a second proxy handler that matches /api to localhost:<backend-port>/api/. By doing so, the API can be accessed using the same MagicDNS url as the WebUI (by adding e.g. /api/auth/get-session at the end). This removes the need to create a separate Tailscale container just for the backend.