fix: flip board coordinates for Black-to-move and prevent coord overflow

[SKOHscripts]

Summary

• Coordinate flip (Black to move): When the board is rotated 180° because it's Black's turn, the rank/file labels are now mirrored too. Previously the bottom-left square showed a1's labels while it actually displayed h8. A flip flag is threaded from applyFenToBoard (via a data-flip attribute) through parseChess into ChessBoard, which reverses the column string (abcdefgh → hgfedcba) and the row string (8…1 → 1…8) inside genColorHTML. Applied to both front.html and back.html.
• Coordinate overflow: fitChessBoardToScreen now measures the full table (board and coordinate cells) instead of only the inner board div, so the right-hand coordinates no longer push the board off-screen when shown.
• Sampling default: lowered target_per_theme from 20 to 17 (defaults, docstrings, main() call, and build_apkg.build_full), reducing the number of puzzles per ELO tranche.
• README: updated the two stale target_per_theme=20 references to 17.

Test plan

• pytest tests/build_apkg_test.py — 31 passed
• Verified the flip transform produces hgfedcba (cols) and 1…8 top-to-bottom (rows), matching a 180°-rotated board
• Visual check in Anki: Black-to-move puzzle shows correctly mirrored coordinates
• Visual check: enabling coordinates on a narrow screen no longer overflows the board to the right

https://claude.ai/code/session_01VAUnQCt5CM2TVpRbsQbSBL

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