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/assets/styles/_globals.scss

@@ -11,14 +11,10 @@
     box-sizing: border-box;
 }
 
-$globals-color-hover: $color-primary;
 a {
-    color:inherit;
-    text-decoration: underline;
+    color: inherit;
     cursor: pointer;
-    @include hover-or-touch {
-        color: $globals-color-hover;
-    }
+    @include flat-button($disabled: false);
 }
 
 body {