feat: roller coaster logo and working favicon

- public/logo.svg: amber coaster silhouette (lift hill + vertical loop +
  camelback) on transparent background; used in header and README
- app/icon.tsx: PNG favicon via Next.js ImageResponse (works in all
  browsers including Safari); renders simplified hill + loop on dark
  rounded-square background
- app/page.tsx: logo img added next to "Thoosie Calendar" title in header
- README.md: logo displayed at top of document
- Remove app/icon.svg (replaced by icon.tsx → PNG)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/icon.tsx

@@ -0,0 +1,59 @@
+import { ImageResponse } from "next/og";
+
+export const size = { width: 32, height: 32 };
+export const contentType = "image/png";
+
+export default function Icon() {
+  return new ImageResponse(
+    (
+      <div
+        style={{
+          width: 32,
+          height: 32,
+          borderRadius: 8,
+          backgroundColor: "#0c1220",
+          display: "flex",
+          position: "relative",
+        }}
+      >
+        {/* Ground line */}
+        <div
+          style={{
+            position: "absolute",
+            left: 3,
+            right: 3,
+            bottom: 7,
+            height: 2,
+            backgroundColor: "#f59e0b",
+            borderRadius: 1,
+          }}
+        />
+        {/* Lift hill — semicircle bump */}
+        <div
+          style={{
+            position: "absolute",
+            left: 3,
+            bottom: 9,
+            width: 10,
+            height: 13,
+            backgroundColor: "#f59e0b",
+            borderRadius: "50% 50% 0 0",
+          }}
+        />
+        {/* Vertical loop — circle outline */}
+        <div
+          style={{
+            position: "absolute",
+            right: 5,
+            bottom: 7,
+            width: 12,
+            height: 12,
+            border: "2.5px solid #f59e0b",
+            borderRadius: "50%",
+          }}
+        />
+      </div>
+    ),
+    { width: 32, height: 32 },
+  );
+}