Add throw origin, direction, and strength options

[djustamante]

Summary

• New throwOrigin config — named zones (top, bottom, left, right, topLeft, topRight, bottomLeft, bottomRight, center) or explicit [x, y, z] world coords. Defaults to 'random' (unchanged behavior).
• New throwDirection config — optional [x, y, z] vector; dice are thrown in that direction instead of toward the opposite side.
• New throwStrength config — per-roll multiplier on throw velocity, clamped by maxThrowStrength (default 3).
• All three are accepted both in the constructor (updateConfig) and as per-roll options in roll/add/reroll.
• Added drag-demo/ (run with npm run demo:drag) — drag across the canvas to throw: origin = drag start, direction = drag vector, strength scales with drag length.

Test plan

• npm install && npm run demo:drag boots the dev server
• Tap (no drag) triggers the default random throw
• Drag in each cardinal direction launches dice in the dragged direction from the drag origin
• Longer drags fling harder; ceiling enforced by maxThrowStrength
• Existing API is unchanged — bare Dice.roll('4d6') behaves as before