[WC-3537]: Fix resizing issue with Signature pad

[r0b1n]

Bug fixes

Strokes stop registering after widget resize

When the widget container was resized mid-stroke (e.g., opening browser DevTools), signature_pad left its internal _drawingStroke flag as true permanently. The pointerup listener had been removed from the window, so strokeEnd never fired and every subsequent pointerdown was silently dropped. Fixed by calling pad.off() before resizing the canvas and pad.on() after — this resets _drawingStroke and re-registers the pointer listeners.

Canvas initializes at wrong size

The canvas was initialized at the HTML default size (300×150) when the ResizeObserver fired before imageSource became available. On init, the pad now reads the container's actual dimensions directly before instantiating SignaturePad.

Code quality improvements

• useSignaturePad owns the ResizeObserver — the hook now calls useResizeObserver internally and returns a containerRef alongside canvasRef. SizeContainer no longer manages the observer or exposes an onResize callback; it is a plain forwardRef component. This removes the prop-threading indirection, and the parentElement lookup.
• pad.redraw() replaces manual toData()/clear()/fromData() — the library's own method handles the snapshot internally.
• Pen options moved to a lookup table — replaced a useMemo with if/else chains with a Record<PenTypeEnum, Options> constant and a getPenOptions helper; type safety is enforced at the call site.
• isSignatureInitialized ref removed — redundant alongside the existing signaturePadRef === null guard.
• Clear-on-reset condition simplified — collapsed three nested if blocks into a single expression.