Fix GPU memory leak in WebGpuViewer when jumping pages

[Danidukiyu]

Fixes #3863

Summary

When using the page navigator slider to rapidly jump through a chapter on the new WebGpu renderer, pages that were queued and subsequently evicted from the WebGpuViewer cache before they finished decoding were leaking their GPU textures.

This PR adds a missing imagePage.cleanup() call in decodeReaderPage to ensure that if a page finishes decoding but has already been evicted from the cache, the newly allocated texture is properly destroyed.

• Tested: Yes (Compiled and verified on physical hardware to confirm the massive GC pressure, freezing, and OOM crash are resolved).
• Self-Reviewed: Yes.
• Visual Changes: No UI changes, strictly memory management.