fix: enable element selection inside same-origin iframes

[liuxiaopai-ai]

Fixes #71.

Summary

• Extend element hit-testing to traverse into same-origin iframes (while safely ignoring cross-origin frames).
• Expand drag-selection querying to include same-origin iframe documents.
• Keep behavior unchanged for cross-origin iframes (no access attempted beyond try/catch).

Notes

This change only supports same-origin iframes because browser security prevents accessing cross-origin iframe DOM.

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