docs: refresh bilingual README and screenshots

[Willxup]

Summary

• Make README.md the English primary document and add README.zh.md as the Simplified Chinese companion.
• Align both documents with the concise ImageSilo structure for screenshots, features, quick start, design boundaries, development, and documentation.
• Replace the light and dark dashboard screenshots with current deterministic demo captures and remove embedded metadata.

Verification

• make check
• make frontend-e2e
• Public API documentation updated when applicable (not applicable)
• No real configuration, credential, Cookie, database, backup, log, or private address added
• Commits include Signed-off-by (DCO)

Compatibility

None. Documentation and screenshots only; no configuration, database, API, container, or upgrade behavior changes.