Add cover thumbnail and chapter list drawer to audio mini-bar

- Store cover and chapters array in audioStore (populated by AudioPlayer on startPlayback)
- Page server returns full chapters list on normal path; empty array on preview
- Mini-bar: replace arrow link with book cover thumbnail (fallback book icon)
- Mini-bar track info area is now a button that toggles a chapter list drawer
- Chapter drawer slides up above the mini-bar, lists all chapters with current one highlighted

ui/src/lib/components/AudioPlayer.svelte

@@ -59,6 +59,8 @@
 		cover?: string;
 		/** Next chapter number, or null/undefined if this is the last chapter. */
 		nextChapter?: number | null;
+		/** Full chapter list for the book (number + title). Written into the store. */
+		chapters?: { number: number; title: string }[];
 		/** List of available voices from the Kokoro API. */
 		voices?: string[];
 	}
@@ -70,6 +72,7 @@
 		bookTitle = '',
 		cover = '',
 		nextChapter = null,
+		chapters = [],
 		voices = []
 	}: Props = $props();
 
@@ -405,6 +408,8 @@
 		audioStore.chapter = chapter;
 		audioStore.chapterTitle = chapterTitle;
 		audioStore.bookTitle = bookTitle;
+		audioStore.cover = cover;
+		audioStore.chapters = chapters;
 
 		// Update OS media session (lock screen / notification center).
 		setMediaSession();