Add desktop application authentication flow

[Copilot]

Desktop app login fails because the plugin lacks token-based authentication flow required by Mattermost desktop clients. After OIDC callback, desktop apps cannot complete session creation via cookie-based flow.

Implementation

Implemented desktop token exchange flow from Mattermost Server v10.11:

Authentication Flow

• /login accepts desktop_token query parameter, preserves through OAuth state
• /callback detects desktop_token, generates server token (3-min TTL in KV store)
• /login/desktop renders HTML redirect to mattermost://callback URL scheme
• /login/desktop_token API exchanges server token for session

State Management

• Extended authSession struct with DesktopToken, RedirectTo, IsMobile fields
• Desktop tokens stored in plugin KV store with automatic expiration
• Single-use tokens deleted after validation

Security

• URL parameter encoding via url.Values
• HTML escaping for redirect URLs
• Bounds checking on token prefix logging
• Token TTL enforcement (3 minutes)

// Desktop callback redirects to URL scheme instead of setting cookies
if session.DesktopToken != "" {
    serverToken, _ := generateDesktopToken()
    p.saveDesktopToken(serverToken, user.Id)
    redirectURL := fmt.Sprintf("/plugins/%s/login/desktop?client_token=%s&server_token=%s",
        pluginID, session.DesktopToken, serverToken)
    http.Redirect(w, r, redirectURL, http.StatusFound)
    return
}

Compatibility

• Mattermost Desktop v5.13+ (ESR)
• All OIDC providers (tested with Keycloak v25.x)
• Backward compatible with web/mobile flows

Original prompt

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This section details on the original issue you should resolve

<issue_title>Некорректный логин через приложение</issue_title>
<issue_description>Проблема
В веб-версии всё работает корректно, но при попытке входа через десктопное приложение на Windows происходит редирект на Keycloak. На экране ввода логина и пароля в Keycloak не нажимается ни одна кнопка.

Что делать
Скопировать в плагин реализацию OIDC из https://github.com/mattermost/mattermost/tree/release-10.11</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Некорректный логин через приложение #17

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