Fix button inconsistencies and macOS layout shifts

This commit fixes layout shifts experienced in macOS Safari when
hovering over top menu items. Instead of making text bold — which was
causing layout shifts — the hover effect now changes the text color.
This ensures a consistent UI across different browsers and platforms.

Additionally, this commit fixes the styling of the privacy button
located in the bottom right corner. Previously styled as an `<a>`
element, it is now correctly represented as a `<button>`.

Furthermore, the commit enhances HTML conformity and accessibility by
correctly using `<button>` and `<a>` tags instead of relying on click
interactions on `<span>` elements.

This commit introduces `FlatButton` Vue component and a new
`flat-button` mixin. These centralize button usage and link styles,
aligning the hover/touch reactions of buttons across the application,
thereby creating a more consistent user interface.

src/presentation/components/TheFooter/TheFooter.vue

@@ -32,8 +32,12 @@
           </a>
         </div>
         <div class="footer__section__item">
-          <AppIcon class="icon" icon="user-secret" />
-          <a @click="showPrivacyDialog()">Privacy</a>
+          <FlatButton
+            label="Privacy"
+            icon="user-secret"
+            flat
+            @click="showPrivacyDialog()"
+          />
         </div>
       </div>
     </div>
@@ -50,6 +54,7 @@ import {
 import ModalDialog from '@/presentation/components/Shared/Modal/ModalDialog.vue';
 import AppIcon from '@/presentation/components/Shared/Icon/AppIcon.vue';
 import { injectKey } from '@/presentation/injectionSymbols';
+import FlatButton from '@/presentation/components/Shared/FlatButton.vue';
 import DownloadUrlList from './DownloadUrlList.vue';
 import PrivacyPolicy from './PrivacyPolicy.vue';
 
@@ -59,6 +64,7 @@ export default defineComponent({
     PrivacyPolicy,
     DownloadUrlList,
     AppIcon,
+    FlatButton,
   },
   setup() {
     const { info } = injectKey((keys) => keys.useApplication);
@@ -99,7 +105,6 @@ export default defineComponent({
 
 .icon {
   margin-right: 0.5em;
-  text-decoration: none;
 }
 
 .footer {