fix(ios): deliver canvas finger-drag (on_drag) — parity with Android

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Problem

On iOS, a Canvas's on_drag handle did nothing. The NIF wires props["on_drag"]
to node.onDrag (mob_nif.m), but the SwiftUI MobCanvasView rendered the draw
ops and attached no drag recognizer, so node.onDrag was never invoked. The
only gesture wiring (mobGestures) covers long-press / double-tap / swipe — not
continuous drag. Net result: finger-drawing worked on Android (MobCanvas had
detectDragGestures) but was dead on iOS.

This surfaced building a finger-drawing screen: drags onto the canvas produced
nothing on iOS.

Fix

Add a canvas-scoped DragGesture(minimumDistance: 0) to MobCanvasView that
calls node.onDrag(dx, dy, x, y, phase) with began / dragging / ended
phases (a @State flag distinguishes the first sample). The Canvas frame is
sized to the declared logical units (canvasWidth/canvasHeight) and draw ops
are drawn in that same space, so the gesture's .local location is already in
canvas coordinates — no pixel→logical rescale is needed (unlike Android,
where Compose hands back pixels). Only attached when node.onDrag != nil, so
non-interactive canvases are unaffected.

Verification

Verified on a physical iPhone (iOS 26.5) via a Sloppy Joe finger-drawing
screen — a canvas plus a color picker and thickness control:

• real-finger drags route through node.onDrag → mob_send_drag →
  {:drag, tag, %{x, y, dx, dy, phase}} and render strokes live;
• per-stroke color and width are preserved;
• taps on the color/thickness buttons and drags on the canvas coexist.

Pairs with the Android side, which already had this via detectDragGestures.

Docs

CHANGELOG.md [Unreleased] → Fixed.

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