fix: drawing tools use pointer events for mouse+touch support

- InteractionEngine: pointerdown on canvas, pointermove/pointerup on document

- Pointer capture for smooth dragging outside canvas

- CSS: touch-action: none on canvas to prevent browser touch interception

- Fixes: tools not working on desktop (mouseup lost outside canvas) and mobile (no touch support)

frontend/src/styles.css

@@ -1458,7 +1458,7 @@ a:hover { text-decoration: underline; }
   .status-item.hide-mobile { display: none; }
 
   /* SVG canvas: ensure it's scrollable/zoomable on touch */
-  .canvas-svg { touch-action: pinch-zoom pan-x pan-y; }
+  .canvas-svg { touch-action: none; }
 }
 
 /* Very small screens (<480px) */