Restore webview API and add OHOS runtime detection

[SWangHash]

• Restore browser_webview_eval and browser_get_url implementations with OHOS conditionals
• Add get_runtime_environment command to detect OHOS environment
• Frontend uses iframe fallback when webview overlay is not supported on OHOS

Summary

Fixes #

Type and Areas

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Areas:

Motivation / Impact

Verification

Reviewer Notes

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.