feat: Enhance layout editor with matrix editing, index display, and key insertion

[yoichiro]

Summary

• Matrix position editor on keycaps: Selected keys show ▲▼◀▶ arrows to change row/col values. Arrows skip to nearest available position to prevent duplicates.
• Layout index display: Each keycap shows its layout array index (0, 1, 2...) so users can see the keymap.c correspondence.
• Key insertion with index: Add Key dialog now includes an Index field to insert keys at specific positions, shifting existing keys. Defaults: index=keyCount (append), row/col=max+1.
• Remove undo/redo: Removed since it was incompatible with 2-way binding auto-apply.
• Add Japanese translations for new UI labels.

Test plan

• Select a key → verify ▲▼◀▶ arrows appear with row,col display
• Click ▶ → verify col jumps to nearest available value (skipping occupied)
• Click ▲ → verify row decreases to nearest available value (min 0)
• Verify all keycaps show layout index numbers (0, 1, 2...)
• Click Add Key → verify Index field with default=keyCount, Row/Col default=max+1
• Add key at index 0 → verify it inserts at beginning and other keys shift
• Add key at last index → verify it appends at end
• Verify matrix changes persist after switching to Code Editor and back
• Verify Undo/Redo buttons are removed

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