feat(seo): add a generated 1200x630 OG image (Gitea #4)

Social-sharing previews had no image — layout.tsx's OG/Twitter tags
existed but pointed at nothing. Adds src/app/opengraph-image.tsx using
next/og's ImageResponse to render a branded card (paw mark, wordmark,
tagline) at request time instead of shipping a static asset, and
switches twitter.card to summary_large_image now that there's an
image for it to show.

src/app/layout.tsx

@@ -25,7 +25,7 @@ export const metadata: Metadata = {
     type: "website",
   },
   twitter: {
-    card: "summary",
+    card: "summary_large_image",
     title: "PawFeed",
     description: "The social platform where pets are the stars.",
   },

src/app/opengraph-image.tsx

@@ -0,0 +1,105 @@
+import { ImageResponse } from "next/og";
+
+export const alt = "PawFeed — the social platform where pets are the stars.";
+export const size = { width: 1200, height: 630 };
+export const contentType = "image/png";
+
+// Simple 1-big-pad + 4-toes paw print, built from plain positioned circles —
+// Satori's CSS support doesn't need raw SVG for a shape this simple.
+function PawMark({ scale = 1, opacity = 1 }: { scale?: number; opacity?: number }) {
+  const s = (n: number) => n * scale;
+  return (
+    <div style={{ position: "relative", width: s(340), height: s(340), opacity, display: "flex" }}>
+      <div
+        style={{
+          position: "absolute",
+          left: s(70),
+          top: s(140),
+          width: s(200),
+          height: s(170),
+          borderRadius: "50% 50% 45% 45%",
+          background: "#FFFFFF",
+        }}
+      />
+      <div style={{ position: "absolute", left: s(30), top: s(30), width: s(90), height: s(110), borderRadius: "50%", background: "#FFFFFF" }} />
+      <div style={{ position: "absolute", left: s(140), top: s(0), width: s(90), height: s(115), borderRadius: "50%", background: "#FFFFFF" }} />
+      <div style={{ position: "absolute", left: s(250), top: s(30), width: s(90), height: s(110), borderRadius: "50%", background: "#FFFFFF" }} />
+      <div style={{ position: "absolute", left: s(-20), top: s(120), width: s(80), height: s(100), borderRadius: "50%", background: "#FFFFFF" }} />
+    </div>
+  );
+}
+
+export default function Image() {
+  return new ImageResponse(
+    (
+      <div
+        style={{
+          width: "100%",
+          height: "100%",
+          display: "flex",
+          background: "linear-gradient(135deg, #FB923C 0%, #F97316 45%, #C2410C 100%)",
+          position: "relative",
+        }}
+      >
+        {/* Decorative oversized paw mark bleeding off the right edge — depth/layering */}
+        <div style={{ position: "absolute", right: -60, top: -40, display: "flex" }}>
+          <PawMark scale={2.05} opacity={0.16} />
+        </div>
+
+        {/* Content column */}
+        <div
+          style={{
+            display: "flex",
+            flexDirection: "column",
+            justifyContent: "center",
+            paddingLeft: 96,
+            paddingRight: 96,
+            width: "100%",
+            height: "100%",
+          }}
+        >
+          <div style={{ display: "flex", alignItems: "center", marginBottom: 28 }}>
+            <PawMark scale={0.62} />
+          </div>
+          <div
+            style={{
+              display: "flex",
+              fontSize: 132,
+              fontWeight: 800,
+              color: "#FFFFFF",
+              letterSpacing: -3,
+              lineHeight: 1,
+            }}
+          >
+            PawFeed
+          </div>
+          <div
+            style={{
+              display: "flex",
+              fontSize: 40,
+              fontWeight: 500,
+              color: "#FFF7ED",
+              marginTop: 22,
+              maxWidth: 820,
+            }}
+          >
+            Where pets are the stars.
+          </div>
+          <div
+            style={{
+              display: "flex",
+              fontSize: 26,
+              fontWeight: 600,
+              color: "#FFEDD5",
+              marginTop: 44,
+              letterSpacing: 1,
+            }}
+          >
+            pawfeed.org
+          </div>
+        </div>
+      </div>
+    ),
+    { ...size }
+  );
+}