Custom fonts vanished from the Font panel after an app restart unless a
book was opened first. The custom-font store is hydrated only by the
reader's FoliateViewer (on book open) or by useReplicaPull (gated on a
signed-in user), so opening Settings straight from the library left the
store empty.
Add a useCustomFonts hook that loads persisted custom fonts on mount,
unconditional of auth or book state, and mount it on the library page.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Huang Xin