[fix] simple theme: style password inputs like text inputs

The AI summary API key is a password input, the preferences stylesheet
only styled input[type="text"] -- the field rendered with the browser
default (white) instead of the theme background.

The description of the field is shortened to what a user needs to decide
whether to fill it in.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

client/simple/src/less/preferences.less

@@ -35,7 +35,8 @@ table {
     width: 300px;
   }
 
-  input[type="text"] {
+  input[type="text"],
+  input[type="password"] {
     width: 13.25rem;
     color: var(--color-toolkit-input-text-font);
     border: none;
@@ -65,7 +66,8 @@ table {
     width: 15em;
 
     select,
-    input[type="text"] {
+    input[type="text"],
+    input[type="password"] {
       font-size: inherit !important;
       margin-top: 0;
       .ltr-margin-right(1rem);