fix(a11y): active sidebar item contrast (WCAG AA)

Starlight's default uses --sl-color-text-accent (our teal #00d4aa) as
the active-item background, which gives ~3:1 contrast vs the dark
inverted text — under WCAG AA (4.5:1). Override with primary blue
+ white text for ~9:1.

Note: customCss loads before Starlight's component CSS, so the rule
needs (0,2,1) specificity to win against Starlight's scoped (0,2,0).

src/styles/anhh-tokens.css

@@ -83,3 +83,24 @@
 body {
   line-height: 1.6;
 }
+
+/*
+ * Active sidebar item — Starlight default uses --sl-color-text-accent as the
+ * background, which on our teal (#00d4aa) gave only ~3:1 contrast against the
+ * dark gray text. Override with our primary blue + white text so contrast
+ * exceeds WCAG AA in both modes.
+ *
+ * Starlight scopes its rules via Astro's data-astro-cid-* attributes, so we
+ * need at least matching specificity. `:is(html)` adds a class-equivalent.
+ */
+/*
+ * Specificity bump: customCss loads BEFORE Starlight's component-scoped CSS,
+ * and Starlight's rule has specificity (0,2,0) via its scoped class. We use
+ * a doubled attribute selector + html prefix for (0,2,1) to override.
+ */
+html [aria-current='page'][aria-current='page'],
+html [aria-current='page'][aria-current='page']:hover,
+html [aria-current='page'][aria-current='page']:focus {
+  background-color: var(--anhh-primary);
+  color: #ffffff;
+}