Remove obsolete tests and add new login page layout

- Deleted test files for NavLink, Navbar, NavbarBrand, Progress, Row, SearchBar, Select, Spinner, Textarea, and a basic test for Button.
- Introduced a new EmptyLayout component for consistent layout structure.
- Added a new login page with a layout that includes a login form and an ASCII art header.
- Updated CSS variables for improved styling consistency across components.
- Removed Vitest configuration file as it is no longer needed.

public/logo.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" aria-label="Nixi – Logo">
+  <defs>
+    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#9f75ff"/>
+      <stop offset="100%" stop-color="#6b04fd"/>
+    </linearGradient>
+  </defs>
+  <g fill="none" stroke="url(#g)" stroke-width="8" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M16 48V16"/>
+    <path d="M48 48V16"/>
+    <path d="M16 16L48 48"/>
+  </g>
+</svg>