Add English README and app previews

[maddylaneeee]

What changed

• rewrites the primary README in English for international discovery
• preserves the full Chinese README as README.zh-CN.md
• adds language switching at the top of both documents
• adds two real screenshots from the current 1.3.6 macOS app
• puts the developer beta call to action, differentiators, supported engines, and current limitations near the top
• links beta feedback to the newly enabled GitHub Discussions and Issues

Why

The previous README was Chinese-first and mostly implementation-oriented. New visitors could not quickly see the actual interface, understand the product's positioning, or learn that this is a source-build developer beta without a notarized binary.

User and developer impact

International visitors get a concise English landing page, Chinese documentation remains available, and beta testers get a clear feedback path. The screenshots contain only non-private test transcript content.

Validation

• both screenshot assets are valid 1271×768 JPEG files
• all new relative links resolve inside the repository
• git diff --check passes
• the source code is unchanged; the existing macOS CI remains the build validation path