feat: profile stats, discover history, end-of-chapter sleep, rating-ranked deck

**Profile stats tab**
- New Stats tab on /profile page (Profile / Stats switcher)
- Reading overview: chapters read, completed, reading, plan-to-read counts
- Activity cards: day streak + avg rating given
- Favourite genres (top 3 by frequency across library/progress)
- getUserStats() in pocketbase.ts — computes streak, shelf counts, genre freq

**Discover history tab**
- New History tab on /discover with full voted-book list
- Per-entry: cover thumbnail, title link, author, action label (Liked/Skipped/etc.)
- Undo button: optimistic update + DELETE /api/discover/vote?slug=...
- Clear all history button; tab shows vote count badge
- getVotedBooks(), undoDiscoveryVote() in pocketbase.ts

**Rating-ranked discovery deck**
- getBooksForDiscovery now sorts by community avg rating before returning
- Tier-based shuffle: books within the same ±0.5 star bucket are still randomised
- Higher-rated books surface earlier without making the deck fully deterministic

**End-of-chapter sleep timer**
- New cycle option: Off → End of Chapter → 15m → 30m → 45m → 60m → Off
- sleepAfterChapter flag in AudioStore; layout handles it in onended (skips auto-next)
- Button shows "End Ch." label when active in this mode

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

ui/src/routes/api/discover/vote/+server.ts

@@ -1,6 +1,6 @@
 import { json, error } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
-import { upsertDiscoveryVote, clearDiscoveryVotes, saveBook } from '$lib/server/pocketbase';
+import { upsertDiscoveryVote, clearDiscoveryVotes, undoDiscoveryVote, saveBook } from '$lib/server/pocketbase';
 
 const VALID_ACTIONS = ['like', 'skip', 'nope', 'read_now'] as const;
 type Action = (typeof VALID_ACTIONS)[number];
@@ -22,9 +22,16 @@ export const POST: RequestHandler = async ({ request, locals }) => {
 	return json({ ok: true });
 };
 
-export const DELETE: RequestHandler = async ({ locals }) => {
+// DELETE /api/discover/vote          → clear all (deck reset)
+// DELETE /api/discover/vote?slug=... → undo single vote
+export const DELETE: RequestHandler = async ({ url, locals }) => {
+	const slug = url.searchParams.get('slug');
 	try {
-		await clearDiscoveryVotes(locals.sessionId, locals.user?.id);
+		if (slug) {
+			await undoDiscoveryVote(locals.sessionId, slug, locals.user?.id);
+		} else {
+			await clearDiscoveryVotes(locals.sessionId, locals.user?.id);
+		}
 	} catch {
 		error(500, 'Failed to clear votes');
 	}