Plan: Fix drag-to-select stopping when scrolling to second page

Problem Analysis

• Understand the issue: Drag selection stops working when scrolling to the second page
• Identify root cause: Gesture detectors are attached per-item in SliverChildBuilderDelegate, only working for mounted items
• Review maintainer comments: Need to refactor to calculate positions globally without relying on item mounting state

Solution Approach

• Move gesture detection from per-item to a global overlay/listener
• Update AssetGridDragSelectionCoordinator to calculate target indices from global positions
• Modify assetsGridBuilder to use a single gesture detector wrapper instead of per-item detectors
• Fix Scrollable context issue - Get ScrollableState from scroll controller instead of BuildContext
• Ensure auto-scrolling continues to work correctly
• Test the changes with the example app (requires Flutter environment)

Implementation Steps

• Create a wrapper widget for the entire grid with gesture detection
• Update coordinator to handle selections purely based on calculated indices
• Remove per-item gesture detectors
• Fix assertion error by accessing ScrollableState from scroll controller's position
• Verify drag selection works across multiple pages (pending Flutter environment)
• Run existing tests to ensure no regressions (pending Flutter environment)

Changes Made

1. asset_grid_drag_selection_coordinator.dart

• Added _calculateIndexFromPosition() method (lines 56-144) to calculate asset index from global position coordinates
• Updated onSelectionStart() to:
  • Accept BoxConstraints instead of index and asset
  • Get ScrollableState from gridScrollController.position.context.notificationContext instead of Scrollable.maybeOf(context)
  • Handle case where auto-scroller may not be available gracefully
• Removed _checkScrollableStatePresent() method as it's no longer needed
• Refactored onSelectionUpdate() to use the shared _calculateIndexFromPosition() method

2. asset_picker_builder_delegate.dart

• Wrapped the entire CustomScrollView with a single GestureDetector (lines 1498-1568)
• Removed per-item gesture detectors from sliverGrid builder

Bug Fix (Latest Commit)

Issue: Assertion failure when drag gesture starts because Scrollable.maybeOf(context) returns null

• The context passed to gesture handlers was from outside the CustomScrollView, so no Scrollable ancestor was found
• Solution: Access ScrollableState directly from the ScrollController's position context instead of relying on BuildContext
• This works because the scroll controller is attached to the CustomScrollView and has access to its internal context

• Fixes [BUG] 拖拽选择到第二页会停止 #695

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