feat: profile page, admin pages, infinite scroll on browse

- Add /profile page with reading settings (voice, speed, auto-next) and password change form
- Add /admin/scrape page showing scraping task history with live status polling and trigger controls
- Add /admin/audio page showing audio cache entries with client-side search filter
- Add changePassword(), listAudioCache(), listScrapingTasks() to pocketbase.ts
- Add /api/admin/scrape and /api/browse-page server-side proxy routes
- Replace browse page pagination with IntersectionObserver infinite scroll
- Update nav: username becomes a /profile link; admin users see Scrape and Audio cache links

ui/src/routes/+layout.svelte

@@ -222,7 +222,26 @@
 				Discover
 			</a>
 				<div class="ml-auto flex items-center gap-4">
-					<span class="text-zinc-400 text-sm hidden sm:block">{data.user.username}</span>
+					{#if data.user?.role === 'admin'}
+						<a
+							href="/admin/scrape"
+							class="text-sm transition-colors {page.url.pathname.startsWith('/admin/scrape') ? 'text-zinc-100 font-medium' : 'text-zinc-400 hover:text-zinc-100'}"
+						>
+							Scrape
+						</a>
+						<a
+							href="/admin/audio"
+							class="text-sm transition-colors {page.url.pathname.startsWith('/admin/audio') ? 'text-zinc-100 font-medium' : 'text-zinc-400 hover:text-zinc-100'}"
+						>
+							Audio cache
+						</a>
+					{/if}
+					<a
+						href="/profile"
+						class="text-sm transition-colors {page.url.pathname === '/profile' ? 'text-zinc-100 font-medium' : 'text-zinc-400 hover:text-zinc-100'}"
+					>
+						{data.user.username}
+					</a>
 					<form method="POST" action="/logout">
 						<button
 							type="submit"