fix: add nil guards to prevent nil-pointer panics

- document.go: check if resp is nil before calling resp.Status() in
  Refresh(), since Playwright's Reload() can return a nil response
- archive.go: check SelectFirst() results for nil before calling
  Type() and Click(), preventing panics when DOM elements are missing

Closes #10, #11

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

sites/archive/archive.go

@@ -104,15 +104,25 @@ func (c Config) Archive(ctx context.Context, b extractor.Browser, target string)
 		return nil, fmt.Errorf("failed to open url: %w", err)
 	}
 
-	err = doc.SelectFirst("input[name='url']").Type(u.String())
+	urlInput := doc.SelectFirst("input[name='url']")
+	if urlInput == nil {
+		_ = doc.Close()
+		return nil, fmt.Errorf("failed to find url input element")
+	}
 
+	err = urlInput.Type(u.String())
 	if err != nil {
 		_ = doc.Close()
 		return nil, fmt.Errorf("failed to type url: %w", err)
 	}
 
-	err = doc.SelectFirst("form#submiturl input[type=\"submit\"]").Click()
+	submitBtn := doc.SelectFirst("form#submiturl input[type=\"submit\"]")
+	if submitBtn == nil {
+		_ = doc.Close()
+		return nil, fmt.Errorf("failed to find submit button")
+	}
 
+	err = submitBtn.Click()
 	if err != nil {
 		_ = doc.Close()
 		return nil, fmt.Errorf("failed to click submit: %w", err)