fix: chapter content vanishes — replace stale untrack snapshots with $derived

html and fetchingContent were captured with untrack() at mount time.
When SvelteKit re-ran the page load (triggered by the layout's settings PUT),
data.html updated but html stayed stale. The {#key} block in the layout then
destroyed and recreated the component, and on remount data.html was momentarily
empty so html became '' and the live-scrape fallback ran unnecessarily.

Fix:
- html is now $derived(scrapedHtml || data.html || '') — always tracks load
- scrapedHtml is a separate $state only set by the live-scrape fallback
- fetchingContent starts false; the fallback sets it true only when actually fetching
- translationStatus/translatingLang: dropped untrack() so they also react to re-runs
- Removed unused untrack import

ui/src/routes/books/[slug]/chapters/[n]/+page.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-	import { onMount, untrack, getContext } from 'svelte';
+	import { onMount, getContext } from 'svelte';
 	import { browser } from '$app/environment';
 	import { goto } from '$app/navigation';
 	import { page } from '$app/state';
@@ -11,8 +11,9 @@
 
 	let { data }: { data: PageData } = $props();
 
-	let html = $state(untrack(() => data.html));
-	let fetchingContent = $state(untrack(() => !data.isPreview && !data.html));
+	let scrapedHtml = $state('');           // only set by the live-preview fallback
+	let html = $derived(scrapedHtml || data.html || '');
+	let fetchingContent = $state(false);
 	let fetchError = $state('');
 	let audioProRequired = $state(false);
 
@@ -202,8 +203,8 @@
 		{ code: 'pt', label: 'PT' },
 		{ code: 'fr', label: 'FR' }
 	];
-	let translationStatus = $state(untrack(() => data.translationStatus ?? 'idle'));
-	let translatingLang = $state(untrack(() => data.lang ?? ''));
+	let translationStatus = $state(data.translationStatus ?? 'idle');
+	let translatingLang = $state(data.lang ?? '');
 	let pollingTimer: ReturnType<typeof setTimeout> | null = null;
 
 	function currentLang() {
@@ -293,6 +294,7 @@
 
 		// If the normal path returned no content, fall back to live preview scrape
 		if (!data.isPreview && !data.html) {
+			fetchingContent = true;
 			(async () => {
 				try {
 					const res = await fetch(
@@ -302,7 +304,7 @@
 					const d = (await res.json()) as { text?: string };
 					if (d.text) {
 						const { marked } = await import('marked');
-						html = await marked(d.text, { async: true });
+						scrapedHtml = await marked(d.text, { async: true });
 					} else {
 						fetchError = m.reader_audio_error();
 					}