Fix Tetris mobile: split controls + fix grid sizing

[samoletovs]

Fixes two mobile issues reported by the kids:

Controls — split into left/right thumb clusters:

• Left hand: ← (left) and ↓ (down), stacked vertically
• Right hand: 2×2 grid — ↑ rotate, → right, ⤓ hard drop, ⇆ hold
• Auto-repeat when holding left/right/down (200ms delay, 60ms interval)
• Switched from click to pointerdown for instant response

Grid sizing — pieces now fill the full board:

• fitCanvas() now JS-driven: measures play-area and side panels, computes correct board width/height, sets board.style.width/height explicitly
• draw() uses separate cellH = h/ROWS alongside cellW = w/COLS so pieces are rectangular cells, not squares clipped in the middle
• Particles on line-clear also use cellH for correct y-position

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