feat: show book counts per library and lens in sidebar

client/src/components/AppSidebar.vue

@@ -113,6 +113,9 @@ onMounted(() => {
                 <span :class="activeLibraryId === lib.id ? 'font-medium text-sidebar-foreground' : 'text-sidebar-foreground/70'">
                   {{ lib.name }}
                 </span>
+                <span v-if="lib.bookCount !== undefined" class="ml-auto shrink-0 rounded-full bg-sidebar-foreground/10 px-2 py-0.5 text-xs tabular-nums text-sidebar-foreground/70 group-data-[collapsible=icon]:hidden">
+                  {{ lib.bookCount.toLocaleString() }}
+                </span>
               </SidebarMenuButton>
             </SidebarMenuItem>
           </SidebarMenu>
@@ -146,6 +149,9 @@ onMounted(() => {
                 <span :class="activeLensId === lens.id ? 'font-medium text-sidebar-foreground' : 'text-sidebar-foreground/70'">
                   {{ lens.name }}
                 </span>
+                <span v-if="lens.bookCount !== undefined" class="ml-auto shrink-0 rounded-full bg-sidebar-foreground/10 px-2 py-0.5 text-xs tabular-nums text-sidebar-foreground/70 group-data-[collapsible=icon]:hidden">
+                  {{ lens.bookCount.toLocaleString() }}
+                </span>
               </SidebarMenuButton>
             </SidebarMenuItem>
             <SidebarMenuItem v-if="lenses.length === 0">

client/src/features/lens/composables/useLenses.ts

@@ -10,6 +10,7 @@ export interface Lens {
   filter: GroupRule | null
   defaultSort: SortSpec[]
   isPublic: boolean
+  bookCount?: number
   createdAt: string
   updatedAt: string
 }