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/page.tsx

@@ -78,14 +78,18 @@ export default async function HomePage({ searchParams }: PageProps) {
           justifyContent: "space-between",
           gap: 12,
         }}>
-          <span style={{
-            fontSize: "1.1rem",
-            fontWeight: 700,
-            color: "var(--color-text)",
-            letterSpacing: "-0.02em",
-          }}>
-            Thoosie Calendar
-          </span>
+          <div style={{ display: "flex", alignItems: "center", gap: 10 }}>
+            {/* eslint-disable-next-line @next/next/no-img-element */}
+            <img src="/logo.svg" alt="" style={{ height: 28, width: "auto" }} />
+            <span style={{
+              fontSize: "1.1rem",
+              fontWeight: 700,
+              color: "var(--color-text)",
+              letterSpacing: "-0.02em",
+            }}>
+              Thoosie Calendar
+            </span>
+          </div>
 
           <span style={{
             background: "var(--color-surface)",