support preview annotations and automation in iframes

[tarik02]

What Changed

• Run the existing preview annotation preload in every frame.
• Render annotation outlines inside their owning frame while the main frame keeps the toolbar and editor.
• Relay nested-frame annotation state through parent frames with postMessage, including coordinate projection and clipping.
• Traverse Electron WebFrameMain.framesInSubtree directly for preview snapshots and MCP actions.
• Expose frameIndex for explicit frame targeting while selector actions search all frames by default.

Why

Preview stopped at iframe boundaries. Selection, regions, annotations, and MCP actions only worked in the main document.

This keeps iframe support inside the existing preview manager and preload. It does not add a frame runtime, annotation worker, global IPC redesign, broker redesign, or webview lifecycle layer.

UI Changes

Iframe elements can be selected and annotated as part of the same preview. Frame-local visuals scroll and clip with their content. The main editor hides when its targets leave the visible preview.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes