Fix mobile tap selection double-trigger (Issue #54)

[ysgrProgramming]

Summary

Fixes Issue #54: On touch devices, selecting a piece causes the selection highlight to flash briefly and immediately disappear due to double-triggering from synthetic click events.

Root Cause

• handleTouchEnd invokes handleSquareClick to emulate a tap
• After the touch ends, the browser dispatches a synthetic click event, which also triggers handleSquareClick via the onClick handler
• The second invocation treats it as "tap same square twice" and deselects

Solution

• Added touchHandledRef to track when touch events are handled
• Call event.preventDefault() in handleTouchEnd when handling taps to prevent synthetic click
• Ignore synthetic click events in onClick handler when touchHandledRef.current is true
• Reset the flag after 300ms (typical synthetic click delay window)

Changes

• src/components/ChessBoard.tsx: Added touch handling flag and preventDefault logic
• tests/components/ChessBoard.responsive.test.tsx: Added tests for selection persistence and double-tap deselection

Testing

• ✅ All existing tests pass (233 tests)
• ✅ New tests verify selection persists after tap without double-trigger
• ✅ New test verifies deselection works correctly with double tap
• ✅ Lint passes with no errors

Self-Walkthrough

Requirements Mapping

1. Prevent synthetic click from firing after touch tap: ✅ Implemented via event.preventDefault() in handleTouchEnd
2. Verify touch taps keep selection active: ✅ Tested via new test should maintain selection after tap without double-triggering from synthetic click
3. Ensure desktop click/drag behavior unchanged: ✅ Verified - desktop clicks are not affected since touchHandledRef is only set for touch events
4. Update responsive touch tests: ✅ Added tests for selection-only scenario

Code Logic

• Touch event handling: When a tap is detected (small movement), we set touchHandledRef.current = true and call event.preventDefault() to prevent synthetic click
• Click event handling: The onClick handler checks touchHandledRef.current and returns early if true, preventing double-trigger
• Flag reset: The flag is reset after 300ms to allow normal click handling for subsequent interactions
• Touch start reset: The flag is also reset on handleTouchStart to ensure clean state for each touch sequence

Closes #54