fix: compute --on-accent in inline head script for all pages

--on-accent was only set by deferred theme-core.js module, causing
all fab--primary and btn--primary elements to have wrong text/icon
color on first paint. Now computed from HSL values in the synchronous
theme-init-script partial, matching theme-core.js logic exactly.

Remove hardcoded fallback from fab--label CSS.

src/partials/theme-init-script.html

@@ -26,6 +26,30 @@
             var L = Number(l);
             if (Number.isFinite(L)) d.style.setProperty("--theme-l", String(Math.min(70, Math.max(20, L))));
           }
+          var hVal = h !== null ? h : 210;
+          var sVal = s !== null ? s : 90;
+          var lVal = l !== null ? Math.min(70, Math.max(20, Number(l))) : 40;
+          var H = (((hVal % 360) + 360) % 360) / 360;
+          var S = Math.min(100, Math.max(0, sVal)) / 100;
+          var Lv = Math.min(100, Math.max(0, lVal)) / 100;
+          var r, g, b;
+          if (S === 0) { r = g = b = Lv; }
+          else {
+            var q = Lv < 0.5 ? Lv * (1 + S) : Lv + S - Lv * S;
+            var p = 2 * Lv - q;
+            var hue2rgb = function(p, q, t) {
+              if (t < 0) t += 1; if (t > 1) t -= 1;
+              if (t < 1/6) return p + (q - p) * 6 * t;
+              if (t < 1/2) return q;
+              if (t < 2/3) return p + (q - p) * (2/3 - t) * 6;
+              return p;
+            };
+            r = hue2rgb(p, q, H + 1/3);
+            g = hue2rgb(p, q, H);
+            b = hue2rgb(p, q, H - 1/3);
+          }
+          var lum = 0.2126 * r + 0.7152 * g + 0.0722 * b;
+          d.style.setProperty("--on-accent", lum > 0.58 ? "hsl(220 14% 12%)" : "hsl(0 0% 98%)");
           var rad = t.radius;
           if (rad === "soft" || rad === "medium" || rad === "strong") {
             d.dataset.radius = rad;