Fix error propagation: surface API validation messages in the UI

- UseJsonExceptionHandler now maps InvalidOperationException to 400 (was 500),
  so upstream business-rule rejections reach the browser as actionable messages.
- CvMatcherController forwards Refit 4xx bodies from cv-matcher-api instead
  of swallowing them in a generic 502.
- ErrorResponse.Score removed; CaptchaController puts the score in Detail.
- Frontend extractApiError helper reads the server Error/error/title field for
  4xx responses and falls back to a generic i18n string for 5xx / missing body.
- All four failure handlers (CV upload, CV match, contact form, subscribe form)
  updated to use extractApiError with the correct rate-limit i18n key.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Apis/api/Controllers/CaptchaController.cs

@@ -70,7 +70,7 @@ namespace Api.Controllers
                 {
                     Error = "Captcha verification failed.",
                     Code = "captcha_verification_failed",
-                    Score = verdict.Score
+                    Detail = verdict.Score.HasValue ? $"Score: {verdict.Score:0.00}" : null
                 });
             }