feat: master feed card (#15)

* feat: master feed card

* fix: adjust tests

frontend/src/components/letterfeed/MasterFeedCard.tsx

@@ -0,0 +1,39 @@
+"use client"
+
+import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card"
+import { Rss, ExternalLink } from "lucide-react"
+import { getMasterFeedUrl } from "@/lib/api"
+
+export function MasterFeedCard() {
+  const feedUrl = getMasterFeedUrl()
+
+  return (
+    <Card className="mb-8">
+      <CardHeader>
+        <CardTitle className="flex items-center gap-2">
+          <Rss className="w-5 h-5 text-orange-500" />
+          Master Feed
+        </CardTitle>
+      </CardHeader>
+      <CardContent className="space-y-4">
+        <p className="text-sm text-muted-foreground">
+          This feed contains all entries from all your newsletters in one place.
+        </p>
+        <div>
+          <h4 className="text-sm font-medium text-gray-700 mb-2">RSS Feed URL</h4>
+          <div className="flex items-center gap-2">
+            <a
+              href={feedUrl}
+              target="_blank"
+              rel="noopener noreferrer"
+              className="inline-flex items-center gap-1 text-sm text-blue-600 hover:text-blue-800 hover:underline"
+            >
+              <ExternalLink className="w-3 h-3" />
+              {feedUrl}
+            </a>
+          </div>
+        </div>
+      </CardContent>
+    </Card>
+  )
+}