feat: add ReactiveStatManager for markdown template support

Introduce ReactiveStatManager to allow dynamic stat replacement within
rendered markdown content. This component scans the article DOM for
`${key}` patterns and reactively updates them based on the journal
stream.

As part of this change, the SVG-based StatSheet system is removed in
favor of this more flexible text-based approach.

src/components/index.ts

@@ -15,6 +15,7 @@ import "./md-yarn-spinner";
 export { Article } from "./Article";
 export type { ArticleProps } from "./Article";
 export { RevealManager } from "./RevealManager";
+export { ReactiveStatManager } from "./journal/ReactiveStatManager";
 export { MobileSidebar, DesktopSidebar } from "./Sidebar";
 export type { SidebarProps } from "./Sidebar";
 export { FileTreeNode, HeadingNode } from "./FileTree";