Replace PAT with github login - #8

Summary

The personal access token sign-in is replaced by signing in with a GitHub account through the OAuth device flow. The user gets a short code, approves it on github.com, and the app receives an access token without any password or token being typed or any client secret shipped.

What changed

• New sign-in flow in Kontext.GitHub (GitHubDeviceLogin, response parsing in DeviceFlowResponses): requests a device code, polls for the token with the interval GitHub returns (handles pending, slow down, expiry, denial and cancellation), only talks to github.com/login/device/code and github.com/login/oauth/access_token, and only accepts an https github.com verification link. New contract IGitHubDeviceLogin and record DeviceCodeChallenge.
• ShellViewModel: the token field is gone; signing in shows the code and the verification link, can be cancelled, and signing out cancels a pending sign-in. The token is saved only after GitHub has accepted it.
• Sign-in screen: the token box and the "create a token" link are replaced by a "Sign in with GitHub" button, the device code panel and a Cancel button.
• Token storage: the access token is stored in the existing Windows Credential Locker entry under a new name, and a previously saved personal access token is deleted on the next start, so existing users sign in once with GitHub.
• The OAuth App client id (not a secret) is read from the environment variable KONTEXT_GITHUB_CLIENT_ID. Without it, the sign-in screen shows a clear "not configured" message. README updated (requirements and a Configuration section).

Before this works end to end

1. Register a GitHub OAuth App at github.com/settings/developers and tick "Enable Device Flow" (no client secret or callback URL needed).
2. Set KONTEXT_GITHUB_CLIENT_ID to its client id and restart Kontext.
3. Organisations that restrict OAuth apps must approve the app, which is different from a personal access token.
   Signing out only forgets the token locally; the grant can be revoked in GitHub settings.

Testing

• dotnet build Kontext.slnx -c Release: 0 warnings, 0 errors.
• dotnet test Kontext.slnx -c Release: all projects passed (GitHub 77, Presentation 203, Architecture 27, Composition 19, GitAutomation 111, and the rest). 71 new tests cover the response parsing and the flow with a stubbed HTTP handler and a fake clock (polling, slow down, expiry, denial, cancellation, errors, URI validation, no token in error messages), and 15 new view model tests replace the three removed token tests.
• Not run: the real device flow against github.com (needs a registered OAuth App), the Credential Locker migration (a leftover saved token is deleted and the user is asked to sign in again), the sign-in screen rendering with the Cancel and open-link buttons, and the environment variable wiring in the app project. These have no automated tests in this repo.

Closes #3