feat(reader): chapter comments + readers-this-week count

- CommentsSection now accepts a chapter prop and scopes comments to that chapter
- Chapter reader page mounts CommentsSection with current chapter number
- Book detail page shows rolling 7-day unique reader count badge
- API GET/POST pass chapter param; pocketbase listComments filters by chapter

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/lib/components/CommentsSection.svelte

@@ -6,10 +6,12 @@
 	import * as m from '$lib/paraglide/messages.js';
 	let {
 		slug,
+		chapter = 0,
 		isLoggedIn = false,
 		currentUserId = ''
 	}: {
 		slug: string;
+		chapter?: number;   // 0 = book-level, N = chapter N
 		isLoggedIn?: boolean;
 		currentUserId?: string;
 	} = $props();
@@ -47,7 +49,7 @@
 		loadError = '';
 		try {
 			const res = await fetch(
-				`/api/comments/${encodeURIComponent(slug)}?sort=${sort}`
+				`/api/comments/${encodeURIComponent(slug)}?sort=${sort}${chapter > 0 ? `&chapter=${chapter}` : ''}`
 			);
 			if (!res.ok) throw new Error(`${res.status}`);
 			const data = await res.json();
@@ -85,7 +87,7 @@
 			const res = await fetch(`/api/comments/${encodeURIComponent(slug)}`, {
 				method: 'POST',
 				headers: { 'Content-Type': 'application/json' },
-				body: JSON.stringify({ body: text })
+				body: JSON.stringify({ body: text, ...(chapter > 0 ? { chapter } : {}) })
 			});
 			if (res.status === 401) { postError = 'You must be logged in to comment.'; return; }
 			if (!res.ok) {