fix(cover): add centre scrim to front overlay for KDP text-legibility compliance

Adds a radial-gradient centre darkening pass (~62 % opacity) directly behind the
title/subtitle/author text block so the gold title reads clearly over any
illustration colour — addresses KDP review feedback about unreadable cover text.
Also adds a fourth text-shadow layer to .front-title for extra depth.

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

templates/cover.html

@@ -304,6 +304,10 @@
       width: 100%;
       height: 100%;
       background:
+        /* centre scrim: ensures title/subtitle contrast over any artwork */
+        radial-gradient(ellipse 88% 58% at 50% 50%,
+          rgba(6, 6, 24, 0.62) 0%,
+          transparent 65%),
         /* deep vignette around all edges */
         radial-gradient(ellipse 90% 90% at 50% 50%,
           transparent 20%,
@@ -346,7 +350,8 @@
       text-shadow:
         0 0 0.22in rgba(184,154,78,0.6),
         0 2px 8px rgba(0,0,0,1),
-        0 4px 24px rgba(0,0,0,0.9);
+        0 4px 24px rgba(0,0,0,1),
+        0 8px 48px rgba(0,0,0,1);
       letter-spacing: 0.01em;
     }