Convert stickers.js to PointerEvents API

[Copilot]

Replaces deprecated touch/mouse event handlers in webclient/stickers.js with the unified PointerEvents API, aligning with the pattern already used in canvas.js.

Changes

• Event listeners: Consolidated 6 separate touch/mouse listeners into 4 unified pointer events

  • touchstart + mousedown → pointerdown
  • touchend + mouseup → pointerup
  • touchmove + mousemove → pointermove
  • Added pointercancel for interrupted gestures

• Multi-pointer tracking: Replaced evt.touches array with activePointers array that tracks pointer lifecycle using pointerId

• Pointer capture: Added setPointerCapture()/releasePointerCapture() for reliable tracking across element boundaries

• Helper functions: Renamed getDistanceBetweenTouches → getDistanceBetweenPointers and getAngleBetweenTouches → getAngleBetweenPointers, removed obsolete touchify() shim

• Edge case handling: Added validation for division by zero when pointers overlap, race condition handling for missing pointers, and state preservation during gesture transitions

Before/After

// Before: separate event handlers + conversion shim
function touchify(evt) {
  if (evt.touches === undefined) {
    evt.touches = [ { clientX: evt.clientX, clientY: evt.clientY } ];
  }
}

sticker.addEventListener("touchstart", move_start);
sticker.addEventListener("mousedown",  move_start);
// ... 4 more listeners

// After: unified pointer events
sticker.addEventListener("pointerdown", move_start);
sticker.addEventListener("pointerup",   move_end);
sticker.addEventListener("pointermove", do_move);
sticker.addEventListener("pointercancel", move_end);

Original prompt

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This section details on the original issue you should resolve

<issue_title>Use PointerEvents for stickers.js</issue_title>
<issue_description>webclient/stickers.js still uses old-style touch/mouse events, convert these to PointerEvents</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Use PointerEvents for stickers.js #76

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