theme: brighten normal dark-mode text above the muted gray level

After the gray text was raised to #cbd5e1 in dark mode, elements using
dark:text-slate-300 (inactive tabs, labels, list text) matched the muted
level. Map dark:text-slate-300 to #e2e8f0 (slate-200, the body's dark
text color) so normal text stays one step brighter than muted gray.
SW cache v96 -> v97.

frontend/style.css

@@ -1793,4 +1793,9 @@ video {
 }
 .dark\:border-slate-800:is(.dark *) {
   border-color: rgb(51 65 85 / var(--tw-border-opacity, 1));
-}
+}
+/* Dark theme: normal (non-muted) UI text — keep one step brighter than the
+   muted gray level so the two remain distinguishable */
+.dark\:text-slate-300:is(.dark *) {
+  color: rgb(226 232 240 / var(--tw-text-opacity, 1));
+}