Add image search metadata: CC0 license data, sitemap, robots, 404 page

Every tile image gets ImageObject license data so search engines can list it
as Creative Commons, the build writes sitemap.xml with all tile images,
robots.txt points at it, unknown paths get a real 404 instead of the gallery,
and the page gets a descriptive title, canonical, large image previews and
an og.png collage for link previews.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/404.html

@@ -0,0 +1,25 @@
+<!doctype html>
+<html lang="en">
+<head>
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width,initial-scale=1">
+<title>not found · vibegif</title>
+<meta name="robots" content="noindex">
+<link rel="icon" type="image/webp" href="/favicon.webp">
+<script>try{localStorage.bg&&document.documentElement.classList.add("dark")}catch{}</script>
+<style>
+:root{--bg:#fff;--fg:#000;--mute:#888}
+.dark{--bg:#000;--fg:#fff;--mute:#777}
+@font-face{font-family:"Stain";src:url("https://cdn.jsdelivr.net/gh/multipleof4/stain.otf@master/dist/Stain.otf") format("opentype");font-display:swap}
+body{background:var(--bg);color:var(--fg);font-family:"Stain",sans-serif;text-align:center;margin:0;padding:15vh 16px}
+h1{font-weight:400;font-size:clamp(22px,5vw,44px);margin:16px 0}
+a{color:var(--mute);font-size:clamp(16px,3vw,22px)}
+a:hover{color:var(--fg)}
+</style>
+</head>
+<body>
+<img src="/ghost/float.webp" width="180" height="180" alt="ghost float">
+<h1>nothing here</h1>
+<a href="/">back to the gallery</a>
+</body>
+</html>

public/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://vibegif.direct-img.link/sitemap.xml