feat: fhirlint.yml config check page

[predictor2718]

Goal

A dedicated page where users can paste a fhirlint.yml and validate it — catches typos, unknown keys, and invalid enum values before committing the config to a repo.

UI

• Monaco Editor (YAML mode) for config input
• "Check config" button
• Results panel: errors with line numbers and suggestions (e.g. "did you mean fhir-version?")

API

POST /api/config/check
Content-Type: application/json

{ "config": "fhir-versoin: 4.0.1\nseverity: verbose\n" }

Response mirrors /api/validate structure:

{
  "valid": false,
  "issues": [
    { "line": 1, "key": "fhir-versoin", "message": "unknown key (did you mean \"fhir-version\"?)" },
    { "line": 2, "key": "severity",     "message": "invalid value \"verbose\" (allowed: information, warning, error)" }
  ]
}

Dependency

Blocked on fhirlint/fhirlint#94 — the backend logic lives there. Once fhirlint config check is implemented as a library call, this page can be wired up.