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/content/partials/support-link.mdx

@@ -1,5 +1,5 @@
 import { URLS } from './urls.mdx';
 
 <div class="anhh-accent-callout">
-  **Still stuck?** Open a [support ticket]({URLS.whmcsTicket}) and our team will help.
+  **Still stuck?** Open a <a href={URLS.whmcsTicket}>support ticket</a> and our team will help.
 </div>