docs(docs): add new problem documentation pages and source selector component

apps/docs/sources.ts

@@ -0,0 +1,34 @@
+/**
+ * The site's content sources — what the sidebar's source selector switches
+ * between (`components/SourceSidebar.astro`), Fumadocs' "root folders". A
+ * source owns every route under its `path`, and the sidebar shows only the
+ * active source's pages. The source at `/` owns whatever no other source
+ * claims, which is how the LeetCode campaign keeps its routes (`/tree-dfs/…`)
+ * while a later source mounts beside it.
+ *
+ * Adding one: a content folder at the path (`content/system-design/`) with an
+ * `index.mdx`, so the selector's link is a page, plus a row here. The active
+ * source is the deepest `path` the current route sits under.
+ */
+export interface Source {
+  label: string;
+  description: string;
+  /** A Blume built-in icon name. */
+  icon: string;
+  path: string;
+}
+
+export const SOURCES: Source[] = [
+  {
+    label: "LeetCode",
+    description: "Data structures & algorithms",
+    icon: "binary",
+    path: "/",
+  },
+  {
+    label: "System Design",
+    description: "Distributed systems interviews",
+    icon: "network",
+    path: "/system-design",
+  },
+];