refactor: handle DNS whitelists

Introduce a single DNSListChecker struct with flags to avoid code
duplication with already existing RBL checker.

web/src/lib/components/WhitelistCard.svelte

@@ -0,0 +1,62 @@
+<script lang="ts">
+    import type { BlacklistCheck } from "$lib/api/types.gen";
+    import { theme } from "$lib/stores/theme";
+
+    interface Props {
+        whitelists: Record<string, BlacklistCheck[]>;
+    }
+
+    let { whitelists }: Props = $props();
+</script>
+
+<div class="card shadow-sm" id="dnswl-details">
+    <div class="card-header" class:bg-white={$theme === "light"} class:bg-dark={$theme !== "light"}>
+        <h4 class="mb-0 d-flex justify-content-between align-items-center">
+            <span>
+                <i class="bi bi-shield-check me-2"></i>
+                Whitelist Checks
+            </span>
+            <span class="badge bg-info text-white">Informational</span>
+        </h4>
+    </div>
+    <div class="card-body">
+        <p class="text-muted small mb-3">
+            DNS whitelists identify trusted senders. Being listed here is a positive signal, but has
+            no impact on the overall score.
+        </p>
+
+        <div class="row row-cols-1 row-cols-lg-2">
+            {#each Object.entries(whitelists) as [ip, checks]}
+                <div class="col mb-3">
+                    <h5 class="text-muted">
+                        <i class="bi bi-hdd-network me-1"></i>
+                        {ip}
+                    </h5>
+                    <table class="table table-sm table-striped table-hover mb-0">
+                        <tbody>
+                            {#each checks as check}
+                                <tr>
+                                    <td title={check.response || "-"}>
+                                        <span
+                                            class="badge"
+                                            class:bg-success={check.listed}
+                                            class:bg-dark={check.error}
+                                            class:bg-secondary={!check.listed && !check.error}
+                                        >
+                                            {check.error
+                                                ? "Error"
+                                                : check.listed
+                                                  ? "Listed"
+                                                  : "Not listed"}
+                                        </span>
+                                    </td>
+                                    <td><code>{check.rbl}</code></td>
+                                </tr>
+                            {/each}
+                        </tbody>
+                    </table>
+                </div>
+            {/each}
+        </div>
+    </div>
+</div>

web/src/lib/components/index.ts

@@ -24,3 +24,4 @@ export { default as SpamAssassinCard } from "./SpamAssassinCard.svelte";
 export { default as SpfRecordsDisplay } from "./SpfRecordsDisplay.svelte";
 export { default as SummaryCard } from "./SummaryCard.svelte";
 export { default as TinySurvey } from "./TinySurvey.svelte";
+export { default as WhitelistCard } from "./WhitelistCard.svelte";