Fix transcriber.NewClient call: add missing MistralAPIKey parameter

The NewClient function expects 5 parameters (groqAPIKey, mistralAPIKey,
whisperModel, aiModel, aiPrompt) but was being called with only 4.

Fixes build error: not enough arguments in call to transcriber.NewClient
  have (string, string, string, string)
  want (string, string, string, string, string)

Co-authored-by: jahruz67 <jahruz67@users.noreply.github.com>

app.go

@@ -543,6 +543,7 @@ func (a *App) SaveSettings(settings map[string]interface{}) string {
 	// Since GetSettings masks the key, we only update if it's not still the masked value.
 	a.transcriber = transcriber.NewClient(
 		a.config.GroqAPIKey,
+		a.config.MistralAPIKey,
 		a.config.WhisperModel,
 		a.config.AIModel,
 		a.config.AIPrompt,