feat: implement inventory CRUD UI components (#18 #19 #20 #21)

Week 2 core inventory management:

**Composables:**
- useInventory: Full CRUD operations for inventory items
- useUnits: Unit fetching and conversion helpers
- useTags: Tag fetching and category filtering

**Components:**
- InventoryList (#18): Grid view with loading/empty/error states
- InventoryCard: Item card with image, quantity controls, tags, expiry
- AddItemForm (#19): Form with tag picker, unit selector, validation
- EditItemModal (#20): Modal form for editing existing items
- Delete functionality (#21): Confirm dialog + cascade tag cleanup

**Features:**
- Quantity quick-actions (+/- buttons on cards)
- Auto-delete when quantity reaches zero
- Expiry date tracking with color-coded warnings
- Tag selection by category in add form
- Responsive grid layout (1-4 columns)
- Product image display from barcode cache
- Form validation and loading states

Closes #18, #19, #20, #21

app/composables/useInventory.ts

@@ -0,0 +1,201 @@
+import type { Database } from '~/types/database.types'
+
+type InventoryItem = Database['public']['Tables']['inventory_items']['Row']
+type InventoryItemInsert = Database['public']['Tables']['inventory_items']['Insert']
+type InventoryItemUpdate = Database['public']['Tables']['inventory_items']['Update']
+
+export const useInventory = () => {
+  const supabase = useSupabase()
+  const { user } = useSupabaseAuth()
+
+  /**
+   * Get all inventory items with denormalized data
+   */
+  const getInventory = async () => {
+    const { data, error } = await supabase
+      .from('inventory_items')
+      .select(`
+        *,
+        product:products(*),
+        unit:units(*),
+        tags:item_tags(tag:tags(*))
+      `)
+      .order('created_at', { ascending: false })
+
+    if (error) {
+      console.error('Error fetching inventory:', error)
+      return { data: null, error }
+    }
+
+    return { data, error: null }
+  }
+
+  /**
+   * Get single inventory item by ID
+   */
+  const getInventoryItem = async (id: string) => {
+    const { data, error } = await supabase
+      .from('inventory_items')
+      .select(`
+        *,
+        product:products(*),
+        unit:units(*),
+        tags:item_tags(tag:tags(*))
+      `)
+      .eq('id', id)
+      .single()
+
+    if (error) {
+      console.error('Error fetching item:', error)
+      return { data: null, error }
+    }
+
+    return { data, error: null }
+  }
+
+  /**
+   * Add new inventory item
+   */
+  const addInventoryItem = async (item: Omit<InventoryItemInsert, 'added_by'>) => {
+    if (!user.value) {
+      return { data: null, error: { message: 'User not authenticated' } }
+    }
+
+    const { data, error } = await supabase
+      .from('inventory_items')
+      .insert({
+        ...item,
+        added_by: user.value.id
+      })
+      .select(`
+        *,
+        product:products(*),
+        unit:units(*),
+        tags:item_tags(tag:tags(*))
+      `)
+      .single()
+
+    if (error) {
+      console.error('Error adding item:', error)
+      return { data: null, error }
+    }
+
+    return { data, error: null }
+  }
+
+  /**
+   * Update inventory item
+   */
+  const updateInventoryItem = async (id: string, updates: InventoryItemUpdate) => {
+    const { data, error } = await supabase
+      .from('inventory_items')
+      .update(updates)
+      .eq('id', id)
+      .select(`
+        *,
+        product:products(*),
+        unit:units(*),
+        tags:item_tags(tag:tags(*))
+      `)
+      .single()
+
+    if (error) {
+      console.error('Error updating item:', error)
+      return { data: null, error }
+    }
+
+    return { data, error: null }
+  }
+
+  /**
+   * Delete inventory item
+   */
+  const deleteInventoryItem = async (id: string) => {
+    // First delete associated tags
+    await supabase
+      .from('item_tags')
+      .delete()
+      .eq('item_id', id)
+
+    const { error } = await supabase
+      .from('inventory_items')
+      .delete()
+      .eq('id', id)
+
+    if (error) {
+      console.error('Error deleting item:', error)
+      return { error }
+    }
+
+    return { error: null }
+  }
+
+  /**
+   * Update item quantity (consume or restock)
+   */
+  const updateQuantity = async (id: string, change: number) => {
+    const { data: item, error: fetchError } = await getInventoryItem(id)
+    if (fetchError || !item) {
+      return { data: null, error: fetchError }
+    }
+
+    const newQuantity = Number(item.quantity) + change
+
+    if (newQuantity <= 0) {
+      // Auto-delete when quantity reaches zero
+      return await deleteInventoryItem(id)
+    }
+
+    return await updateInventoryItem(id, { quantity: newQuantity })
+  }
+
+  /**
+   * Add tags to item
+   */
+  const addItemTags = async (itemId: string, tagIds: string[]) => {
+    const items = tagIds.map(tagId => ({
+      item_id: itemId,
+      tag_id: tagId
+    }))
+
+    const { error } = await supabase
+      .from('item_tags')
+      .insert(items)
+
+    if (error) {
+      console.error('Error adding tags:', error)
+      return { error }
+    }
+
+    return { error: null }
+  }
+
+  /**
+   * Remove tag from item
+   */
+  const removeItemTag = async (itemId: string, tagId: string) => {
+    const { error } = await supabase
+      .from('item_tags')
+      .delete()
+      .eq('item_id', itemId)
+      .eq('tag_id', tagId)
+
+    if (error) {
+      console.error('Error removing tag:', error)
+      return { error }
+    }
+
+    return { error: null }
+  }
+
+  return {
+    getInventory,
+    getInventoryItem,
+    addInventoryItem,
+    updateInventoryItem,
+    deleteInventoryItem,
+    updateQuantity,
+    addItemTags,
+    removeItemTag
+  }
+}