feat: Add multi-source font and style manager

Rename md-emfont to md-font with support for Google, emfont, and local
sources. Introduce article-style-manager.ts for centralized lifecycle
management of article-level styles and external stylesheet link
injection with reference counting. Refactor md-bg to use the new
manager, removing direct createEffect on article styles.

src/components/index.ts

@@ -4,7 +4,7 @@ import "./md-table";
 import "./md-link";
 import "./md-pins";
 import "./md-bg";
-import "./md-emfont";
+import "./md-font";
 import "./md-deck";
 import "./md-commander/index";
 import "./md-yarn-spinner";
@@ -22,7 +22,7 @@ export { FileTreeNode, HeadingNode } from "./FileTree";
 export type { DiceProps } from "./md-dice";
 export type { TableProps } from "./md-table";
 export type { BgProps } from "./md-bg";
-export type { EmfontProps } from "./md-emfont";
+export type { FontProps } from "./md-font";
 export type { YarnSpinnerProps } from "./md-yarn-spinner";
 export type { TokenProps } from "./md-token";