Show the actual mobile H5 experience in README

The previous H5 preview no longer represented the current phone browser flow, so the README gallery now uses a composed image from the latest mobile screenshots while preserving the existing asset path and gallery proportions.

Scope-risk: narrow
Confidence: high
Tested: Viewed generated docs/images/desktop_ui/12_h5_access.png and confirmed dimensions are 2960x2100
Not-tested: Full docs build; image-only README asset replacement

Author: NanmiCoder