[BUG] OAuth callback page: broken Cyrillic on Windows (missing charset=utf-8) #217

Description

After a successful OAuth authentication for MCP (streamable-http), the browser displays the callback page at http://127.0.0.1:<port>/callback with Cyrillic characters rendered as mojibake (УÑпешно! instead of "Успешно!").

However, the authentication itself is successful — the issue affects only the HTML rendering.

Environment

• OS: Windows 10/11
• VS Code + Zoo Code extension (ZooCodeOrganization.zoo-code, v3.54.1)
• MCP: remote streamable-http + Keycloak OAuth
• UI Locale: Russian (ru)

Root Cause

In src/services/mcp/utils/callbackServer.ts, the response is returned as:

"Content-Type": "text/html"

... without charset=utf-8, and the <head> section lacks <meta charset="utf-8">.
Strings from t("mcp:oauth.callback.auth_success"), etc., are in UTF-8, but browsers on Windows often interpret them as Windows-1251.

Proposed Fix

1. In all res.writeHead(..., { "Content-Type": ... }) calls for HTML, update the header to:
   "Content-Type": "text/html; charset=utf-8"
2. Add the following tag to the <head> of every HTML template:
   <meta charset="utf-8">

Affected Files

• src/services/mcp/utils/callbackServer.ts (approx. lines 94–180)
• src/i18n/locales/ru/mcp.json → oauth.callback.*

Expected Behavior

The text should render correctly without corruption: "Успешно! MCP-сервер успешно аутентифицирован…".