feat(uat-batch-21): a11y — auth-page link contrast bumped past AA

`text-[#007bff] hover:underline` (light blue, 12-14px) was falling
below WCAG 1.4.3 AA contrast against the auth shell's white card.
Bumped to `text-[#0058b3]` (darker variant of the same hue) and
added `underline underline-offset-2 hover:no-underline` so the link
is always visibly underlined as a backup affordance.

Affects: /login, /reset-password, /set-password, /portal/login,
/portal/forgot-password, portal password-set-form. Button bg colors
(white-text on the same blue) are unchanged — those pass AA at
button sizes.

tsc clean. 1419/1419 vitest pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/(auth)/set-password/page.tsx

@@ -120,7 +120,10 @@ function SetPasswordInner() {
             Please use the link from the email we sent you. If the link is broken, ask your
             administrator for a new one.
           </p>
-          <Link href="/login" className="inline-block text-sm text-[#007bff] hover:underline">
+          <Link
+            href="/login"
+            className="inline-block text-sm text-[#0058b3] underline-offset-2 underline hover:no-underline"
+          >
             Back to sign in
           </Link>
         </div>