fix: Preview resize issues when double-clicking column dividers

[w-ahmad]

Description

Column resizing was previously started immediately in OnPointerPressed when pressing a column divider.

In Preview mode, this initialized the resize preview even when the user was not actually dragging the divider. This caused problems when double-clicking a divider to auto-size a column, including:

• Cell content becoming temporarily or permanently misaligned
• Selection borders having an incorrect width
• Brief visual offsets during double-click auto-sizing

This change delays BeginResize until the pointer actually moves.

OnPointerPressed now only initializes the resize gesture state and captures the pointer. The actual Live/Preview resize starts on the first OnPointerMoved.

This prevents Preview state from being created for simple clicks or double-clicks while preserving the existing behavior for real drag-resizing.

The same behavior is applied when resizing either the current column from its right edge or the previous column from the left edge.

Related Issue

Closes #442

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 📝 Documentation update
• ♻️ Refactor
• 🧪 Test
• 🔧 Chore / maintenance

Checklist

• This PR is not from my main branch
• Tested with WinUI target
• Tested with Uno Platform target
• Unit / integration tests added or updated
• Documentation updated to reflect changes
• Code follows the project's coding conventions