fix(decnet_web/Layout): theme toggle icon stays visible on hover

The global button:hover rule in index.css forces color: var(--bg)
+ matrix-glow on the lucide icon's currentColor stroke, making
the sun/moon icon disappear into the toggle button's tinted
background on hover. Pin color: var(--accent) and box-shadow:
none on .theme-toggle-btn:hover so the icon stays in its base
colour and the button doesn't pick up the wider button-hover
halo.

decnet_web/src/components/Layout.css

@@ -233,6 +233,11 @@
 .theme-toggle-btn:hover {
   background: var(--accent-tint-10);
   border-color: var(--accent);
+  /* Without these the global button:hover wins and pushes
+   * color to var(--bg) and adds matrix-glow — the lucide icon
+   * (currentColor stroke) collapses into the tinted background. */
+  color: var(--accent);
+  box-shadow: none;
 }
 
 .crumbs {