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>