feat(CheckboxFilter): add comprehencive documentation

src/shared/ui/CheckboxFilter/CheckboxFilter.svelte

@@ -10,17 +10,37 @@ import { onMount } from 'svelte';
 import { cubicOut } from 'svelte/easing';
 import { slide } from 'svelte/transition';
 
+/**
+ * CheckboxFilter Component
+ *
+ * A collapsible category filter with checkboxes. Displays selected count as a badge
+ * and supports reduced motion for accessibility. Used in sidebar filtering UIs.
+ *
+ * Design choices:
+ * - Open by default for immediate visibility and interaction
+ * - Badge shown only when filters are active to reduce visual noise
+ * - Transitions use cubicOut for natural deceleration
+ * - Local transition prevents animation when component first renders
+ */
+
 interface CategoryFilterProps {
+    /** Display name for this filter group (e.g., "Categories", "Tags") */
     filterName: string;
+    /** Array of categories with their selection states */
     categories: Category[];
+    /** Callback when a category checkbox is toggled */
     onCategoryToggle: (id: string) => void;
 }
 
 const { filterName, categories, onCategoryToggle }: CategoryFilterProps = $props();
 
+// Toggle state - defaults to open for better discoverability
 let isOpen = $state(true);
+// Accessibility preference to disable animations
 let prefersReducedMotion = $state(false);
 
+// Check reduced motion preference on mount (window access required)
+// Event listener allows responding to system preference changes
 onMount(() => {
     if (typeof window !== 'undefined') {
         const mediaQuery = window.matchMedia('(prefers-reduced-motion: reduce)');
@@ -35,21 +55,24 @@ onMount(() => {
     }
 });
 
-// Optimized animation configuration
+// Animation config respects user preferences - zero duration if reduced motion enabled
+// Local modifier prevents animation on initial render, only animates user interactions
 const slideConfig = $derived({
     duration: prefersReducedMotion ? 0 : 250,
     easing: cubicOut,
 });
 
-// Count selected categories for badge
+// Derived for reactive updates when categories change - avoids recomputing on every render
 const selectedCount = $derived(categories.filter(c => c.selected).length);
 const hasSelection = $derived(selectedCount > 0);
 </script>
 
+<!-- Collapsible card wrapper with subtle hover state for affordance -->
 <Collapsible.Root
     bind:open={isOpen}
     class="w-full rounded-lg border bg-card transition-colors hover:bg-accent/5"
 >
+    <!-- Trigger row: title, expand indicator, and optional count badge -->
     <div class="flex items-center justify-between px-4 py-2">
         <Collapsible.Trigger
             class={buttonVariants({
@@ -61,12 +84,14 @@ const hasSelection = $derived(selectedCount > 0);
         >
             <h4 class="text-sm font-semibold">{filterName}</h4>
 
+            <!-- Chevron rotates based on open state for visual feedback -->
             <div
                 class="shrink-0 transition-transform duration-200 ease-out"
                 style:transform={isOpen ? 'rotate(0deg)' : 'rotate(-90deg)'}
             >
                 <ChevronDownIcon class="h-4 w-4" />
             </div>
+            <!-- Badge only appears when items are selected to avoid clutter -->
             {#if hasSelection}
                 <Badge
                     variant="secondary"
@@ -78,6 +103,7 @@ const hasSelection = $derived(selectedCount > 0);
         </Collapsible.Trigger>
     </div>
 
+    <!-- Expandable content with slide animation -->
     {#if isOpen}
         <div
             transition:slide|local={slideConfig}
@@ -85,6 +111,8 @@ const hasSelection = $derived(selectedCount > 0);
         >
             <div class="px-4 py-3">
                 <div class="flex flex-col gap-2.5">
+                    <!-- Each item: checkbox + label with interactive hover/focus states -->
+                    <!-- Keyed by category.id for efficient DOM updates -->
                     {#each categories as category (category.id)}
                         <Label
                             for={category.id}
@@ -95,6 +123,9 @@ const hasSelection = $derived(selectedCount > 0);
                                 active:scale-[0.98] active:transition-transform active:duration-75
                             "
                         >
+                            <!--
+                                Checkbox handles toggle, styled for accessibility with focus rings
+                            -->
                             <Checkbox
                                 id={category.id}
                                 checked={category.selected}
@@ -106,6 +137,7 @@ const hasSelection = $derived(selectedCount > 0);
                                     focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2
                                 "
                             />
+                            <!-- Label text changes weight/color based on selection state -->
                             <span
                                 class="
                                     text-sm select-none transition-all duration-150 ease-out