feat(docs): replace default Fumadocs OG image with custom branded template

Swap the generic `fumadocs-ui/og/takumi` default for a hand-crafted OG image using the Sofa cinema theme — dark background, amber accent, DM Serif Display title, DM Sans body, and a radial gradient — matching the app's visual identity.

- Load DM Sans (400/500) and DM Serif Display (400) at runtime from Google Fonts via old Safari UA to get TrueType URLs
- Render header with Sofa logo SVG + "Sofa Docs" label, centered title/description block, and an amber accent bar
- Scale title font size based on length (68px → 56px for longer strings)
- Define theme constants (BG, FG, PRIMARY, MUTED) from the app's oklch palette

docs/src/app/og/docs/[...slug]/route.tsx

@@ -1,10 +1,42 @@
 import { ImageResponse } from "@takumi-rs/image-response";
-import { generate as DefaultImage } from "fumadocs-ui/og/takumi";
 import { notFound } from "next/navigation";
 import { getPageImage, source } from "@/lib/source";
 
 export const revalidate = false;
 
+async function loadFont(family: string, weight: number): Promise<ArrayBuffer> {
+  const params = new URLSearchParams({
+    family: `${family}:wght@${weight}`,
+    display: "swap",
+  });
+  const css = await fetch(`https://fonts.googleapis.com/css2?${params}`, {
+    headers: {
+      // Old user-agent to get TrueType format (not woff2)
+      "User-Agent":
+        "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_8; de-at) AppleWebKit/533.21.1 (KHTML, like Gecko) Version/5.0.5 Safari/533.21.1",
+    },
+  }).then((res) => res.text());
+
+  const url = css.match(/src: url\((.+?)\)/)?.[1];
+  if (!url) throw new Error(`Could not load font: ${family} ${weight}`);
+  return fetch(url).then((res) => res.arrayBuffer());
+}
+
+const fonts = Promise.all([
+  loadFont("DM Sans", 400),
+  loadFont("DM Sans", 500),
+  loadFont("DM Serif Display", 400),
+]);
+
+// oklch(0.13 0.006 55)  → background
+const BG = "#090706";
+// oklch(0.93 0.015 80)  → foreground
+const FG = "#ede7dd";
+// oklch(0.8 0.14 65)    → primary / amber accent
+const PRIMARY = "#fba952";
+// oklch(0.63 0.02 80)   → muted foreground
+const MUTED = "#90887c";
+
 export async function GET(
   _req: Request,
   { params }: RouteContext<"/og/docs/[...slug]">,
@@ -13,16 +45,97 @@ export async function GET(
   const page = source.getPage(slug.slice(0, -1));
   if (!page) notFound();
 
+  const [dmSans, dmSansMedium, dmSerif] = await fonts;
+  const { title, description } = page.data;
+
   return new ImageResponse(
-    <DefaultImage
-      title={page.data.title}
-      description={page.data.description}
-      site="My App"
-    />,
+    <div
+      style={{
+        display: "flex",
+        flexDirection: "column",
+        width: "100%",
+        height: "100%",
+        padding: "60px 80px",
+        backgroundColor: BG,
+        backgroundImage: `radial-gradient(ellipse at 80% 0%, rgba(180, 140, 50, 0.07) 0%, transparent 50%)`,
+        fontFamily: "DM Sans",
+        color: FG,
+        border: `1px solid rgba(236, 231, 223, 0.08)`,
+      }}
+    >
+      {/* Header */}
+      <div style={{ display: "flex", alignItems: "center", gap: "12px" }}>
+        <svg width="28" height="28" viewBox="0 0 24 24">
+          <title>Sofa Logo</title>
+          <path
+            fill={PRIMARY}
+            d="M7 12v1h10v-1a3 3 0 0 1 2.993-3a4.6 4.6 0 0 0-.07-.78a4 4 0 0 0-3.143-3.143C16.394 5 15.93 5 15 5H9c-.93 0-1.394 0-1.78.077A4 4 0 0 0 4.077 8.22a4.6 4.6 0 0 0-.07.78A3 3 0 0 1 7 12"
+          />
+          <path
+            fill={PRIMARY}
+            d="M18.444 18H5.556a3.6 3.6 0 0 1-.806-.092V19a.75.75 0 0 1-1.5 0v-1.849A3.55 3.55 0 0 1 2 14.444V12a2 2 0 1 1 4 0v1.2a.8.8 0 0 0 .8.8h10.4a.8.8 0 0 0 .8-.8V12a2 2 0 1 1 4 0v2.444a3.55 3.55 0 0 1-1.25 2.707V19a.75.75 0 0 1-1.5 0v-1.092a3.6 3.6 0 0 1-.806.092"
+          />
+        </svg>
+        <span
+          style={{
+            fontSize: "20px",
+            fontWeight: 500,
+            color: PRIMARY,
+            letterSpacing: "0.02em",
+          }}
+        >
+          Sofa
+        </span>
+      </div>
+
+      {/* Title + description */}
+      <div
+        style={{
+          display: "flex",
+          flex: 1,
+          flexDirection: "column",
+          justifyContent: "center",
+          gap: "20px",
+        }}
+      >
+        <div
+          style={{
+            fontFamily: "DM Serif Display",
+            fontSize: title.length > 30 ? 56 : 68,
+            fontWeight: 400,
+            lineHeight: 1.1,
+            color: FG,
+          }}
+        >
+          {title}
+        </div>
+        {description ? (
+          <div style={{ fontSize: "24px", lineHeight: 1.5, color: MUTED }}>
+            {description}
+          </div>
+        ) : null}
+      </div>
+
+      {/* Accent bar */}
+      <div
+        style={{
+          display: "flex",
+          width: "120px",
+          height: "3px",
+          borderRadius: "2px",
+          background: PRIMARY,
+        }}
+      />
+    </div>,
     {
       width: 1200,
       height: 630,
       format: "webp",
+      fonts: [
+        { name: "DM Sans", data: dmSans, weight: 400 as const },
+        { name: "DM Sans", data: dmSansMedium, weight: 500 as const },
+        { name: "DM Serif Display", data: dmSerif, weight: 400 as const },
+      ],
     },
   );
 }