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>

public/logo.svg

@@ -0,0 +1,19 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 56" fill="none">
+  <!-- Lift hill + ground + camelback -->
+  <path
+    d="M 2 52 L 20 6 L 38 52 L 116 52 Q 134 22 152 52 L 196 52"
+    stroke="#f59e0b"
+    stroke-width="3.5"
+    stroke-linecap="round"
+    stroke-linejoin="round"
+  />
+  <!-- Vertical loop -->
+  <circle
+    cx="75"
+    cy="33"
+    r="19"
+    stroke="#f59e0b"
+    stroke-width="3.5"
+    fill="none"
+  />
+</svg>