feat: add static demo site at demo.snapotter.com

- New apps/demo/ that reuses apps/web components with mocked API layer
- Full UI shell: login, change password, analytics consent, dashboard, all tool pages
- Stateful mock tracks session flow (password change, analytics consent)
- Demo banner with link to GitHub repo
- Processing attempts show info message with GitHub link
- Deployed to Cloudflare Pages as static site (no backend)

Also links demo across all surfaces:
- README: "Live Demo" badge
- Landing navbar: "Try Demo" CTA button (replaces "Book a Demo")
- Landing hero: "No sign-ups. No credit card." tagline
- Docs getting-started: "Try before installing" tip box

Other changes:
- Docs: move NVIDIA GPU section above GHCR, demote GHCR to collapsed details
- Fix before-after slider checkerboard background for transparency
- Fix remove-bg preview reset when no effects applied

apps/demo/src/demo-banner.tsx

@@ -0,0 +1,32 @@
+import { useState } from "react";
+
+export function DemoBanner() {
+  const [dismissed, setDismissed] = useState(false);
+
+  if (dismissed) return null;
+
+  return (
+    <div className="relative z-[9999] flex items-center justify-center gap-3 bg-blue-600 px-4 py-2 text-sm text-white">
+      <span>
+        This is a live demo. Processing is disabled.{" "}
+        <a
+          href="https://github.com/snapotter-hq/SnapOtter"
+          target="_blank"
+          rel="noopener noreferrer"
+          className="font-semibold underline underline-offset-2 hover:text-blue-100"
+        >
+          Self-host SnapOtter
+        </a>{" "}
+        for full functionality.
+      </span>
+      <button
+        type="button"
+        onClick={() => setDismissed(true)}
+        className="ms-2 shrink-0 rounded px-1.5 py-0.5 text-xs font-medium hover:bg-blue-700"
+        aria-label="Dismiss banner"
+      >
+        Dismiss
+      </button>
+    </div>
+  );
+}