fix: use idp variant for resource auth login page icons

[Bolex80]

Bug Description

The resource auth login page (auth/resource/[resourceGuid]) — used for per-site / custom-branded logins like "Authenticate to access [resource]" — shows the generic OIDC icon on the Google and Microsoft SSO buttons instead of their branded logos.

The main login page and the org login page were already fixed, but the resource auth page's else branch was missed.

Fixes #3631

Root Cause

src/app/auth/resource/[resourceGuid]/page.tsx, in the non-SaaS / non-org path (lines ~282-287):

const idpsRes = await priv.get<AxiosResponse<ListIdpsResponse>>("/idp");
loginIdps = idpsRes.data.data.idps.map((idp) => ({
    idpId: idp.idpId,
    name: idp.name,
    variant: idp.type   // <-- always "oidc" for OIDC-backed IdPs (Google, Azure)
})) as LoginFormIDP[];

This path passes idp.type as the icon variant. For OIDC-backed providers the brand lives in the separate idpOidcConfig.variant column ("google" / "azure"), while idp.type is always "oidc". So IdpTypeIcon always resolves to the generic /idp/openid.png.

The /idp list API already returns variant (server/routers/idp/listIdps.ts selects both type and variant). This path simply wasn't using it.

Fix

variant: idp.variant ?? idp.type

This mirrors the identical fix already applied to the main login page (src/app/auth/login/page.tsx) and the org login page (src/app/auth/org/[orgId]/page.tsx) in PR #3637 — the resource-auth page's else branch was the one place left unfixed. idp.variant is nullable in the API schema, so ?? idp.type preserves the previous fallback behavior for providers without a branded variant.

How to Verify

1. Configure Google and Microsoft/Azure OIDC identity providers (both stored with idp.type = "oidc" but idpOidcConfig.variant = "google" / "azure").
2. Open a Pangolin-protected resource's login page (e.g. https://<resource>/auth/login, the branded "Authenticate to access X" page).
3. Confirm the Google button shows the multicolor Google logo and the Microsoft button shows the Azure logo instead of the generic OIDC icon.

Test Plan

• Added regression test for this bug — N/A (existing pattern; no unit test for this mapping on main; the fix matches the already-merged approach in PR Fix: Use variant instead of type only for the Org login IDPs #3637)
• Existing tests still pass — the change is a one-line mapping, consistent with the shipped fix for auth/login
• Manual verification — reproduced and confirmed the identical bug on the resource auth page

Risk Assessment

Low — cosmetic-only change to which logo PNG is rendered. It does not affect authentication, redirect URLs, or IdP configuration. The fallback ?? idp.type preserves current behavior for any provider where variant is null.

[oschwartz10612]

Thanks!