[bugfix] Fix shift+click deselection in asset panel (#8396)

## Summary
Fix shift+click range selection not properly deselecting assets when
selecting a smaller range, and improve selection performance.

## Changes
- **Bug Fix**: Shift+click now replaces selection with the new range
instead of combining with existing selection
- **Performance**: Remove unnecessary `.every()` check in `setSelection`
(O(n) → O(1))
- **Tests**: Add 23 unit tests for asset selection logic

## Test Plan
- [x] Click 1st asset → only 1st selected
- [x] Shift+click 3rd asset → items 1-3 selected
- [x] Shift+click 1st asset → only 1st selected (was broken, now fixed)
- [x] All 23 new unit tests pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8396-bugfix-Fix-shift-click-deselection-in-asset-panel-2f76d73d3650814ca060d1e6a40cf6d4)
by [Unito](https://www.unito.io)

src/platform/assets/composables/useAssetSelectionStore.test.ts

@@ -0,0 +1,138 @@
+import { createPinia, setActivePinia } from 'pinia'
+import { beforeEach, describe, expect, it } from 'vitest'
+
+import { useAssetSelectionStore } from './useAssetSelectionStore'
+
+describe('useAssetSelectionStore', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+  })
+
+  describe('addToSelection', () => {
+    it('adds an asset ID to the selection', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      expect(store.isSelected('asset-1')).toBe(true)
+      expect(store.selectedCount).toBe(1)
+    })
+
+    it('can add multiple IDs', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      store.addToSelection('asset-2')
+      expect(store.selectedCount).toBe(2)
+    })
+  })
+
+  describe('removeFromSelection', () => {
+    it('removes an asset ID from the selection', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      store.removeFromSelection('asset-1')
+      expect(store.isSelected('asset-1')).toBe(false)
+      expect(store.selectedCount).toBe(0)
+    })
+  })
+
+  describe('setSelection', () => {
+    it('replaces entire selection with new IDs', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      store.addToSelection('asset-2')
+
+      store.setSelection(['asset-3', 'asset-4'])
+
+      expect(store.isSelected('asset-1')).toBe(false)
+      expect(store.isSelected('asset-2')).toBe(false)
+      expect(store.isSelected('asset-3')).toBe(true)
+      expect(store.isSelected('asset-4')).toBe(true)
+      expect(store.selectedCount).toBe(2)
+    })
+
+    it('can set to empty selection', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      store.setSelection([])
+      expect(store.selectedCount).toBe(0)
+    })
+  })
+
+  describe('clearSelection', () => {
+    it('clears all selections and resets lastSelectedIndex', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      store.setLastSelectedIndex(5)
+
+      store.clearSelection()
+
+      expect(store.selectedCount).toBe(0)
+      expect(store.lastSelectedIndex).toBe(-1)
+    })
+  })
+
+  describe('toggleSelection', () => {
+    it('adds unselected item', () => {
+      const store = useAssetSelectionStore()
+      store.toggleSelection('asset-1')
+      expect(store.isSelected('asset-1')).toBe(true)
+    })
+
+    it('removes selected item', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      store.toggleSelection('asset-1')
+      expect(store.isSelected('asset-1')).toBe(false)
+    })
+  })
+
+  describe('isSelected', () => {
+    it('returns true for selected items', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      expect(store.isSelected('asset-1')).toBe(true)
+    })
+
+    it('returns false for unselected items', () => {
+      const store = useAssetSelectionStore()
+      expect(store.isSelected('asset-1')).toBe(false)
+    })
+  })
+
+  describe('setLastSelectedIndex', () => {
+    it('updates lastSelectedIndex', () => {
+      const store = useAssetSelectionStore()
+      store.setLastSelectedIndex(10)
+      expect(store.lastSelectedIndex).toBe(10)
+    })
+  })
+
+  describe('reset', () => {
+    it('clears selection and resets index', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      store.setLastSelectedIndex(5)
+
+      store.reset()
+
+      expect(store.selectedCount).toBe(0)
+      expect(store.lastSelectedIndex).toBe(-1)
+    })
+  })
+
+  describe('computed properties', () => {
+    it('hasSelection returns true when items are selected', () => {
+      const store = useAssetSelectionStore()
+      expect(store.hasSelection).toBe(false)
+      store.addToSelection('asset-1')
+      expect(store.hasSelection).toBe(true)
+    })
+
+    it('selectedIdsArray returns array of selected IDs', () => {
+      const store = useAssetSelectionStore()
+      store.addToSelection('asset-1')
+      store.addToSelection('asset-2')
+      expect(store.selectedIdsArray).toContain('asset-1')
+      expect(store.selectedIdsArray).toContain('asset-2')
+    })
+  })
+})