Fix loading-spinner spacing and add logo to header brand

The "Hinzufügen" buttons reserved layout space for their loading
spinner even when idle, since .htmx-indicator only toggled opacity
(not display), leaving a visible gap before the centered text. Switch
to display:none/inline-flex so the spinner takes no space until an
htmx request is in flight, and drop the static plus icon next to
"Hinzufügen" so the idle button just shows centered text with the
spinner appearing in its place while loading.

Also adds the favicon as a small logo image in front of "LinkVault"
in the shared topbar.

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

app/templates/_topbar.html

@@ -1,6 +1,8 @@
 {% import "_icons.html" as icons %}
 <header class="topbar">
-  <a href="/" class="brand" style="text-decoration:none;">Link<span>Vault</span></a>
+  <a href="/" class="brand" style="text-decoration:none;">
+    <img src="/static/favicon.svg" alt="" width="24" height="24" style="vertical-align:-6px; margin-right:6px;">Link<span>Vault</span>
+  </a>
   <div class="topbar-right">
     <nav class="mode-switch">
       <a href="/" class="mode-tab {% if request.url.path == '/' %}active{% endif %}">