Can't add MCP servers that need a static auth header

[matiacone]

Problem

https://api.excalidraw.com/api/v1/mcp returns 401 Authorization header required because it needs a static API key.

Root cause

Chicken-and-egg in the probe:

1. probeEndpoint (packages/plugins/mcp/src/sdk/plugin.ts:466-522) connects with no auth → 401, fails.
2. Falls back to OAuth probe → not supported → fails.
3. Returns "Could not connect to MCP endpoint and no OAuth was detected".

The UI (AddMcpSource.tsx:609) only shows the Authentication section after a successful probe, so the user can't supply the header needed to make the probe succeed.

Fix

Treat 401 as a third probe outcome (requiresHeaderAuth) alongside connected/OAuth. Surface the auth section, let the user pick a secret-backed header, then re-probe with it before enabling "Add source".

Acceptance

• Excalidraw (and other static-token MCPs) add successfully via UI.
• Genuine unreachable errors (DNS, 5xx) still fail cleanly.
• OAuth flow unchanged.