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/login.ejs

@@ -0,0 +1,17 @@
+<div class="form-container">
+  <h1>Sign in</h1>
+  <% if (error) { %>
+  <p class="error"><%= error %></p>
+  <% } %>
+  <form method="POST" action="/login">
+    <label>
+      Username
+      <input type="text" name="username" required autofocus>
+    </label>
+    <label>
+      Password
+      <input type="password" name="password" required>
+    </label>
+    <button type="submit">Login</button>
+  </form>
+</div>