Fix nightly-update checkbox layout and hide the updates block until loaded

The checkbox inherited the global 100%-wide input style, pushing its label
out of the sidebar. App 0.2.17, firmware unchanged.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

screen_manager/app/static/index.html

@@ -74,7 +74,7 @@
             seconden.
           </p>
           <button class="quiet" id="refresh">↻ Vernieuwen</button>
-          <div class="updates" id="updates">
+          <div class="updates" id="updates" hidden>
             <h2>Firmware-updates</h2>
             <p class="hint" id="updates-hint"></p>
             <button id="update-all" hidden>Alle schermen bijwerken</button>

screen_manager/app/static/style.css

@@ -251,7 +251,12 @@ aside h2 {
   cursor: pointer;
 }
 .check input {
-  margin-top: 3px;
+  width: 16px;
+  height: 16px;
+  flex: none;
+  margin: 2px 0 0;
+  padding: 0;
+  accent-color: var(--accent, #2e5bff);
 }
 .check small {
   display: block;