feat: in-reader settings, more like this, reading history, audio filter, feed page

- Add floating gear button + theme/font/size drawer in chapter reader
- Add 'More like this' horizontal scroll row on book detail page
- Add reading history tab on profile page (chronological progress timeline)
- Add audio-available badge + filter toggle on catalogue (GET /api/audio/slugs)
- Fix discover card entry animation pop (split entry vs snap-back cubic-bezier)
- Add dedicated /feed page showing books followed users are reading
- Add Feed nav link (desktop + mobile) in all 5 locales

ui/src/routes/api/audio/slugs/+server.ts

@@ -0,0 +1,16 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { getSlugsWithAudio } from '$lib/server/pocketbase';
+
+/**
+ * GET /api/audio/slugs
+ * Returns the list of book slugs that have at least one completed audio chapter.
+ * Cached for 5 minutes at the CDN/proxy level.
+ */
+export const GET: RequestHandler = async () => {
+	const slugs = await getSlugsWithAudio().catch(() => new Set<string>());
+	return json(
+		{ slugs: [...slugs] },
+		{ headers: { 'Cache-Control': 'public, max-age=300' } }
+	);
+};