Fix text-paper (and all utility colours) being overridden on <a> tags

Global resets (a, button, body, img) were outside any @layer, so
they beat Tailwind's @layer utilities in the cascade. Wrapping them
in @layer base restores the expected order: base < utilities.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/css/main.css

@@ -39,6 +39,7 @@
   color-scheme: light;
 }
 
+@layer base {
   *, *::before, *::after { box-sizing: border-box; }
   html, body { margin: 0; padding: 0; }
 
@@ -57,6 +58,7 @@ img    { display: block; max-width: 100%; }
   button { font: inherit; color: inherit; background: none; border: 0; cursor: pointer; }
   a      { color: inherit; text-decoration: none; }
   ::selection { background: var(--ink); color: var(--paper); }
+}
 
 /* ── paper grain overlay ── */
 body::before {