docs: document GitHub login for Tauri and Android

[hizclick]

Summary

This PR updates the user and technical documentation to explain GitHub authentication in Android and native Tauri builds.

Native builds use GitHub Device Flow instead of the browser OAuth redirect flow. The documentation now clearly distinguishes these authentication methods and provides instructions for users and maintainers.

Changes

• Added step-by-step GitHub login instructions for Android and native desktop applications.
• Documented how to use the temporary device code at github.com/login/device.
• Documented the native Device Flow prompt shown by MainMenu.
• Updated the user interface and GitHub import documentation.
• Documented the technical authentication flow between Vue, Vuex, Tauri, and GitHub.
• Added native authentication configuration requirements.
• Clarified that native builds require GH_APP_CLIENT_ID.
• Clarified that native builds do not use GH_APP_CLIENT_SECRET or GH_APP_CALL_BACK.
• Added the authentication page to the VuePress technical documentation sidebar.
• Added security guidance for entering GitHub device codes.

Updated files

closes issue #173

• docs/.vuepress/config.js
• docs/technical/Auth.md
• docs/technical/MainMenu.md
• docs/user/UserInterface.md
• docs/user/actions.md
• docs/user/installation.md

Motivation

The existing documentation described only browser-based GitHub authentication. On Android and other native Tauri builds, users are not redirected back to Cartographer through an OAuth callback. Instead, they must authorize the application using GitHub Device Flow.

Without dedicated instructions, users may not understand how to use the displayed device code, and maintainers may incorrectly configure a callback URL or client secret for native builds.

Authentication flow

1. The user selects Login with GitHub.
2. Cartographer displays a temporary device code.
3. The user opens https://github.com/login/device.
4. The user enters the code and authorizes Cartographer.
5. Cartographer polls GitHub for authorization.
6. Login completes automatically and the user's GitHub avatar appears.

Validation

• Run npm run docs:dev.
• Review the user login instructions.
• Verify the technical authentication page appears in the sidebar.
• Check internal documentation links.
• Run npm run docs:build.
• Verify Device Flow login on Android or another native Tauri build.

Notes

This PR changes documentation only. It does not change the authentication implementation.