feat(design): add column resize handles to LedgerTable (#463)

[laurentketterle-hub]

Description

Implements column resize for the virtualized Ledger table (#463).

Changes

• Added ResizeHandle component with drag-to-resize functionality
• Mouse drag resizing with visual feedback
• Keyboard accessible (Tab to handle, aria-label)
• Column widths persist via state
• Minimum column width of 80px enforced
• Visual resize indicator (dashed outline) during resize
• Resize handle CSS with hover/active/dragging states

Accessibility

• Resize handles have role="separator", aria-orientation="vertical"
• Keyboard accessible (focusable via Tab)
• No text selection during resize

Testing

• Added LedgerTable.resize.test.tsx with 3 test cases
• Tests verify resize handles render, ARIA roles, and keyboard support

Closes #463

[laurentketterle-hub]

I claim this bounty. PR ready for review.