Introduce a dialog for switching between built-in content and a
user-selected local folder. Persist the folder handle in IndexedDB
so the selection survives page refreshes. Scan the folder for .md,
.yarn, and .csv files to build the file index.
Update App and Sidebar to pass file tree and heading data, and
add TypeScript declarations for the File System Access API.
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.
hyper