feat: replace the epub reader with foliate-js

Drops epub.js's 1600-location pre-pass and its localStorage cache: foliate
hands back a CFI and an overall fraction on every relocate, so first open no
longer freezes.

Also fixes resuming (pre-resolves the stored CFI, falling back to the stored
percentage rather than silently resetting to page one), progress saving
(missing Content-Type, and no flush on the way out), and arrow keys inside
the book iframe.

frontend/src/lib/reader/stylesheet.ts

@@ -16,9 +16,11 @@ export interface ReaderPalette {
  * <style id="chitai-theme"> with the full palette into every page, so the tokens
  * are resolved on documentElement before first paint.
  *
- * Call this after the dark class has landed, not in the same tick as a mode flip.
+ * `dark` is passed in rather than read off the class list, because mode-watcher's
+ * store is the authority and the class trails it by a frame. Call this after that
+ * frame, though — the token values themselves do come from the class.
  */
-export function readReaderPalette(): ReaderPalette {
+export function readReaderPalette(dark: boolean): ReaderPalette {
 	const styles = getComputedStyle(document.documentElement);
 	const token = (name: string, fallback: string) =>
 		styles.getPropertyValue(name).trim() || fallback;
@@ -29,7 +31,7 @@ export function readReaderPalette(): ReaderPalette {
 		fg: token('--foreground', '#000000'),
 		muted: token('--muted-foreground', '#666666'),
 		link: token('--primary', '#0066cc'),
-		dark: document.documentElement.classList.contains('dark')
+		dark
 	};
 }