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/file-view.ejs

@@ -0,0 +1,19 @@
+<div class="file-view">
+  <h1><%= file.original_name %></h1>
+  <% if (file.mime_type.startsWith('image/')) { %>
+  <img src="/f/<%= file.id %>/raw" alt="<%= file.original_name %>">
+  <% } else if (file.mime_type.startsWith('video/')) { %>
+  <video controls src="/f/<%= file.id %>/raw" preload="metadata"></video>
+  <% } else if (file.mime_type.startsWith('audio/')) { %>
+  <audio controls src="/f/<%= file.id %>/raw" preload="metadata"></audio>
+  <% } %>
+  <div class="file-actions">
+    <a href="/f/<%= file.id %>/raw" download="<%= file.original_name %>" class="btn">Download</a>
+    <a href="/f/<%= file.id %>/raw" target="_blank" class="btn">Open</a>
+  </div>
+  <% if (isOwner) { %>
+  <form method="POST" action="/files/<%= file.id %>/delete">
+    <button type="submit" class="danger">Delete</button>
+  </form>
+  <% } %>
+</div>