fix(ui): replace admin fallback with sign-in CTA on error pages

[AAdewunmi]

Summary
Replaces the admin-oriented fallback action on shared error pages with a public sign-in CTA so recovery paths stay role-safe and aligned with the broader ReturnHub product experience.

Changes

• updated templates/errors/403.html to replace Open Django admin with Go to sign in
• updated templates/errors/404.html to replace Open Django admin with Go to sign in
• updated templates/errors/500.html to replace Open Django admin with Go to sign in
• preserved the existing Return to landing page primary action on all shared error templates
• kept the branded error layout, tone, and shared rh-* UI structure intact

Why
The previous fallback action was technically valid, but it exposed an admin-oriented recovery path on shared public error pages used by customers, merchants, ops users, and anonymous visitors. Replacing it with a sign-in CTA keeps the recovery path broadly useful, better matches ReturnHub’s role-entry model, and avoids surfacing Django admin as the default fallback for non-admin audiences.

Validation

• verified templates/errors/403.html, 404.html, and 500.html now point to accounts:login_merchant for the secondary CTA
• confirmed the primary landing-page recovery action remains unchanged
• confirmed the shared error handlers remain wired through ui.error_views and config/urls.py

Result

• shared error pages now present a role-safe recovery action
• the error experience remains branded and consistent with the rest of ReturnHub
• users are directed toward sign-in rather than an admin-only fallback path

Notes

• this change is UX-oriented rather than infrastructural; the previous admin link was wired correctly
• authenticated users still have clear access to allowed surfaces through the shared role-aware app navigation
• if desired later, the sign-in CTA could be made even more context-aware by routing to a surface chooser rather than a single sign-in entry

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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