fix(decnet_web/css): sweep violet rgba literals to tokens

Credentials drawer code-block labels (printable:, b64:) and a
dozen other violet wash/tint sites still carried bare rgba(238,
130, 238, *) literals — bright magenta in light mode where
--violet has resolved to charcoal-purple #2d1b4e. Mirrors the
prior matrix/alert/warn/info sweeps: by-alpha buckets land on
var(--violet-tint-10) or var(--violet).

decnet_web/src/components/Webhooks.css

@@ -282,7 +282,7 @@
 .wh-secret-modal {
   background: var(--secondary-color);
   border: 1px solid var(--violet);
-  box-shadow: 0 0 24px rgba(238, 130, 238, 0.4);
+  box-shadow: 0 0 24px var(--violet);
   padding: 28px;
   max-width: 560px;
   width: 100%;