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/not-found.ejs

@@ -0,0 +1,5 @@
+<div class="form-container">
+  <h1>404 — Not found</h1>
+  <p>The page or file you requested does not exist.</p>
+  <p><a href="/">Go home</a></p>
+</div>