First start: create the config file and open the Config view

Without a config file arr-tui refused to start. It now creates an empty
one (private, with its directory) and opens the Config view. A service
with neither address nor key is "not configured", which validation
allows: its client returns ErrNotConfigured without a request, its
indicator stays grey, it is logged once as information, and its lists
say so instead of showing errors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/sabnzbd/sabnzbd.go

@@ -40,6 +40,10 @@ func NewClient(baseURL, apiKey string) *Client {
 	}
 }
 
+// ErrNotConfigured is returned without a request while the client has no
+// address: the service is not set up.
+var ErrNotConfigured = errors.New("not configured: enter its address and API key in the Config view (F6)")
+
 // Number is a float that SABnzbd sends either as a JSON number or as a string ("0.48").
 type Number float64
 
@@ -172,6 +176,9 @@ func (c *Client) call(ctx context.Context, params url.Values, out any) error {
 	c.mu.RLock()
 	baseURL, apiKey := c.baseURL, c.apiKey
 	c.mu.RUnlock()
+	if baseURL == "" {
+		return ErrNotConfigured
+	}
 	params.Set("apikey", apiKey)
 
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, baseURL+"/api?"+params.Encode(), nil)