add placeholder Lectures page and link it to sidebar [#41]

Author: Dymon18

src/components/Sidebar/Sidebar.astro

@@ -28,7 +28,7 @@ const sections = [
     icon: "mdi mdi-video",
     title: "Lectures",
     disabled: false,
-    link: "https://www.youtube.com/playlist?list=PLY7TEz3ZRQHTnY56q2uJtdXg-bl-c0sDk",
+    link: "/lectures",
   },
   { icon: "mdi mdi-creation", title: "Sandbox", disabled: true },
   // { icon: "mdi mdi-beaker", title: "Experiments", disabled: true },

src/pages/lectures.astro

@@ -0,0 +1,13 @@
+---
+import Back from "@components/Back/Back.astro";
+import { default as Layout } from "src/layouts/Content/Content.astro";
+---
+
+<Layout title="Lectures">
+  <div class="Question__bar">
+    <Back href={`/`} label="Home" />
+  </div>
+
+  <h1>📚 Lectures</h1>
+  <p>This is a placeholder page for archived lectures. YAML integration coming soon.</p>
+</Layout>