improve(dx): fix OG image font download failure

Replace Unicode special chars (crown, sun, infinity, grid) with
colored initials using each game accent color. Eliminates the
"Failed to load dynamic font" build warning from satori/next-og.

app/opengraph-image.tsx

@@ -4,6 +4,14 @@ export const alt = "Puzzle Trainer";
 export const size = { width: 1200, height: 630 };
 export const contentType = "image/png";
 
+const GAMES = [
+  { label: "Q", color: "#d97706" },
+  { label: "T", color: "#ea580c" },
+  { label: "Z", color: "#2563eb" },
+  { label: "S", color: "#16a34a" },
+  { label: "P", color: "#7c3aed" },
+];
+
 export default function OGImage() {
   return new ImageResponse(
     (
@@ -19,39 +27,33 @@ export default function OGImage() {
           fontFamily: "system-ui, -apple-system, sans-serif",
         }}
       >
-        {/* Game icons row */}
         <div style={{ display: "flex", gap: 24, marginBottom: 40 }}>
-          {["♛", "☀", "∞", "#", "▦"].map((icon, i) => (
+          {GAMES.map((g) => (
             <div
-              key={i}
+              key={g.label}
               style={{
                 width: 72,
                 height: 72,
-                background: "#111827",
+                background: g.color,
                 borderRadius: 16,
                 display: "flex",
                 alignItems: "center",
                 justifyContent: "center",
                 fontSize: 32,
-                color: "#f9fafb",
+                fontWeight: 700,
+                color: "#ffffff",
               }}
             >
-              {icon}
+              {g.label}
             </div>
           ))}
         </div>
-
-        {/* Title */}
         <div style={{ fontSize: 72, fontWeight: 700, color: "#111827", letterSpacing: -2 }}>
           Puzzle Trainer
         </div>
-
-        {/* Tagline */}
         <div style={{ fontSize: 28, color: "#6b7280", marginTop: 16 }}>
           5 puzzles logiques · chaque jour
         </div>
-
-        {/* URL */}
         <div
           style={{
             position: "absolute",