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.

tests/e2e/modal-layout-shifts.cy.ts

@@ -16,7 +16,7 @@ describe('Modal interaction and layout stability', () => {
       });
 
       cy
-        .contains('a', 'Privacy')
+        .contains('button', 'Privacy')
         .click();
 
       cy