feat: user profiles, subscriptions, and subscription feed

- PocketBase: new user_subscriptions collection (follower_id, followee_id)
- pocketbase.ts: subscribe/unsubscribe/getFollowingIds/getPublicProfile/
  getUserPublicLibrary/getUserCurrentlyReading/getSubscriptionFeed helpers
- GET /api/users/[username] — public profile with subscription state
- POST/DELETE /api/users/[username]/subscribe — follow/unfollow
- /users/[username] — public profile page: avatar, stats, follow button,
  currently reading grid, full library grid
- CommentsSection: usernames are now links to /users/[username]
- Home page: 'From People You Follow' section powered by subscription feed

ui/src/lib/components/CommentsSection.svelte

@@ -354,7 +354,11 @@
 								<span class="text-[9px] font-semibold text-zinc-300 leading-none">{initials(comment.username)}</span>
 							</div>
 						{/if}
-						<span class="text-sm font-medium text-zinc-200">{comment.username || 'Anonymous'}</span>
+						{#if comment.username}
+						<a href="/users/{comment.username}" class="text-sm font-medium text-zinc-200 hover:text-amber-400 transition-colors">{comment.username}</a>
+					{:else}
+						<span class="text-sm font-medium text-zinc-400">Anonymous</span>
+					{/if}
 						<span class="text-zinc-600 text-xs">&middot;</span>
 						<span class="text-xs text-zinc-500">{formatDate(comment.created)}</span>
 					</div>
@@ -491,7 +495,11 @@
 												<span class="text-[8px] font-semibold text-zinc-300 leading-none">{initials(reply.username)}</span>
 											</div>
 										{/if}
-										<span class="text-xs font-medium text-zinc-300">{reply.username || 'Anonymous'}</span>
+										{#if reply.username}
+										<a href="/users/{reply.username}" class="text-xs font-medium text-zinc-300 hover:text-amber-400 transition-colors">{reply.username}</a>
+									{:else}
+										<span class="text-xs font-medium text-zinc-400">Anonymous</span>
+									{/if}
 										<span class="text-zinc-600 text-xs">&middot;</span>
 										<span class="text-xs text-zinc-500">{formatDate(reply.created)}</span>
 									</div>