chore: replace hand-rolled layout() with @fastify/view + EJS

Convert all src/views/*.ts template-literal modules to .ejs templates
under views/. Register @fastify/view plugin in server.ts with EJS
engine and _layout.ejs as the layout file. Update route handlers to
use reply.view() instead of reply.send(layout(...)). Delete the old
TypeScript view modules and layout.ts.

Closes #19

views/upload-result.ejs

@@ -0,0 +1,9 @@
+<div class="form-container">
+  <h1>File uploaded</h1>
+  <p><strong><%= filename %></strong> is ready to share.</p>
+  <div class="share-box">
+    <input type="text" id="share-url" value="<%= shareUrl %>" readonly>
+    <button onclick="navigator.clipboard.writeText(document.getElementById('share-url').value)">Copy link</button>
+  </div>
+  <p><a href="/upload">Upload another</a> &middot; <a href="/files">My files</a></p>
+</div>