fix(a11y): broken support/signin links + WCAG AA contrast in light mode

Caught by Lighthouse on prod:

1. CRITICAL: support-link.mdx and signing-in.mdx used the MDX expression
   `{URLS.x}` inside Markdown-link parens, which MDX doesn't evaluate —
   the resulting href was URL-encoded '%7BURLS.whmcsTicket%7D' on every
   page with a Support partial. Replaced with HTML anchors so the
   expression evaluates.
2. Light-mode --sl-color-text-accent was the brand teal (#00d4aa) on a
   cream background = 1.73:1 contrast. Introduced --anhh-accent-on-light
   (#047857) at ~6:1 for inline accent text/links. Brand graphics keep
   the original teal.
3. Bumped light-mode --anhh-text-secondary and --anhh-text-muted to
   #334155 / #475569 so muted UI text clears AA.
4. Hub brand link aria-label now includes 'Knowledge Base' so the
   accessible name covers the visible text.

src/styles/anhh-tokens.css

@@ -51,18 +51,26 @@
   --anhh-bg-elevated: #ffffff;
   --anhh-bg-surface: #e2e8f0;
   --anhh-text-primary: #0f172a;
-  --anhh-text-secondary: #475569;
-  --anhh-text-muted: #94a3b8;
+  --anhh-text-secondary: #334155;     /* bumped from #475569 for AA contrast on muted UI text */
+  --anhh-text-muted: #475569;         /* bumped from #94a3b8 — same reason */
   --anhh-border-color: #cbd5e1;
   --anhh-accent-dim: rgba(0, 212, 170, 0.08);
   --anhh-accent-border: rgba(0, 212, 170, 0.3);
 
+  /*
+   * Darker accent for light mode so inline accent-coloured text and links
+   * (Starlight binds --sl-color-text-accent) pass WCAG AA against the bg.
+   * The brand teal #00d4aa is only ~1.7:1 on the cream background; #047857
+   * is ~6:1, which clears AA.
+   */
+  --anhh-accent-on-light: #047857;
+
   --sl-color-bg: var(--anhh-bg);
   --sl-color-bg-nav: var(--anhh-bg-elevated);
   --sl-color-bg-sidebar: var(--anhh-bg-elevated);
   --sl-color-bg-inline-code: var(--anhh-bg-surface);
   --sl-color-text: var(--anhh-text-primary);
-  --sl-color-text-accent: var(--anhh-accent);
+  --sl-color-text-accent: var(--anhh-accent-on-light);
   --sl-color-gray-1: var(--anhh-text-primary);
   --sl-color-gray-2: var(--anhh-text-secondary);
   --sl-color-gray-3: var(--anhh-text-muted);